A positive electrode material, a preparation method thereof and a lithium ion battery

Abstract

This invention provides a cathode material, its preparation method, and a lithium-ion battery. The preparation method includes: (1) mixing an organic ligand, a magnesium source, and a solvent, and reacting to obtain a magnesium-based MOF; wherein the molar ratio of the organic ligand to the magnesium source is (1.8-2.3):1; (2) mixing carbon-coated lithium vanadium oxyphosphate, a solvent, and the magnesium-based MOF, and sintering to obtain the cathode material. In the preparation method of this invention, a carbon layer and a magnesium-based MOF layer are sequentially coated on the surface of lithium vanadium oxyphosphate, which can effectively reduce the contact between the cathode material and the electrolyte, suppress its side reactions with the electrolyte, and improve the cycle stability of the material; and can improve the surface porosity, ionic conductivity, electronic conductivity, and Li-ion battery of the material. + The insertion and extraction speed is increased, thereby improving the polarization phenomenon at high magnification and effectively enhancing the rate performance of the material.

Description

A cathode material, its preparation method, and a lithium-ion battery Technical Field

[0001] This invention belongs to the field of battery technology, and relates to a cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Among various energy storage technologies, developing battery systems with high energy density and long-term cycle stability is a key step in realizing the widespread application of hybrid or electric vehicles, and also a key element in realizing renewable energy technologies.

[0003] Currently, most lithium-ion batteries used in electronic devices or hybrid electric vehicles employ transition metal oxides as positive electrode active materials, such as lithium cobalt oxide, lithium nickel cobalt manganese oxide, or lithium manganese oxide. Among these, the commercially available positive electrode material LiCoO2 is relatively expensive, and it also presents some safety issues in large-scale applications, thus limiting its further development and application. Similarly, lithium nickel cobalt manganese oxide and lithium manganese oxide also have some shortcomings.

[0004] Therefore, XO4 (X = Si, P or S) polyanionic cathode materials with high thermal stability and high safety performance have received widespread attention. The characteristics of this type of material are that the transition metal cations and polyanions form a stable three-dimensional framework, which provides more channels and space for lithium ion insertion and extraction, and can also keep the potential stable.

[0005] CN114572958A provides a fluorinated polyanionic cathode material, the preparation method of which includes: adding a template agent and a fluorinated additive to a spray-dried mother liquor, granulating using spray drying technology, and finally high-temperature solid-state sintering to obtain the fluorinated polyanionic cathode material. CN104064769B provides a lithium vanadium oxyphosphate cathode material, the preparation method of which includes: dissolving lithium vanadium oxyphosphate raw material and a template agent in deionized water; adjusting the pH to 2-5; then placing the above solution in a water bath and stirring until it becomes gel-like; then drying the gel to obtain a lithium vanadium oxyphosphate precursor; then sintering the precursor in an air atmosphere and cooling it to room temperature to obtain the lithium vanadium oxyphosphate cathode material.

[0006] Among them, LiVOPO4 has a high operating voltage and theoretically can de-intercalate and de-intercalate two Li atoms. + Its capacity is as high as 317mAh·g -1 In LiVOPO4, the covalent bond of PO can reduce V. 4+ / V 5+ The antibonding orbital energy provides a higher operating voltage (approximately 4.0V), but it also leads to increased side reactions between the material and the electrolyte, poor material conductivity, poor cycling performance, and excessively rapid capacity decay at high discharge rates.

[0007] Therefore, there is an urgent need for a method to prepare lithium vanadium oxyphosphate to improve the cycling performance and rate performance of the material. Summary of the Invention

[0008] To address the shortcomings of existing technologies, the present invention aims to provide a cathode material, its preparation method, and a lithium-ion battery. In the preparation method of the present invention, a carbon layer and a magnesium-based MOF layer are sequentially coated onto the surface of lithium vanadium phosphate. This effectively reduces the contact between the cathode material and the electrolyte, suppresses side reactions with the electrolyte, and improves the cycle stability of the material. Furthermore, it can improve the surface porosity, ionic conductivity, electronic conductivity, and Li-ion battery properties of the material. + The insertion and extraction speed is increased, thereby improving the polarization phenomenon at high magnification and effectively enhancing the rate performance of the material.

[0009] To achieve this objective, the present invention adopts the following technical solution:

[0010] In a first aspect, the present invention provides a method for preparing a cathode material, the method comprising:

[0011] (1) Mix the organic ligand, magnesium source and solvent, and after reaction, obtain magnesium-based MOF;

[0012] The molar ratio of the organic ligand to the magnesium source is (1.8-2.3):1 (for example, it can be 1.8:1, 1.85:1, 1.9:1, 1.95:1, 2:1, 2.1:1, 2.15:1, 2.2:1, 2.25:1 or 2.3:1, etc.);

[0013] (2) The carbon-coated lithium vanadium oxyphosphate, solvent and the magnesium-based MOF are mixed and sintered to obtain the cathode material.

[0014] This invention provides a method for preparing a cathode material by sequentially coating a carbon layer and a magnesium-based MOF layer onto the surface of lithium vanadium phosphate. This effectively reduces the contact between the cathode material and the electrolyte, suppresses side reactions with the electrolyte, stabilizes the material structure, and improves the cycle stability of the material. Furthermore, it can reduce the solid-solid interface resistance between the materials and the solid-liquid interface resistance between the material and the electrolyte, and improve the surface porosity, ionic conductivity, electronic conductivity, and Li-2O3 content of the material. + The insertion and extraction speed is increased, thereby improving the polarization phenomenon at high magnification and effectively enhancing the rate performance of the material.

[0015] In this invention, when the molar ratio of organic ligand to magnesium source is too low, the synthesized MOF will be less, that is, the coating of lithium vanadium oxyphosphate main material will be less, the side reaction between electrolyte and main material will increase, and the cycle performance will be poor. When the molar ratio of organic ligand to magnesium source is too high, the impurity phase will be formed, and the side reaction with electrolyte will make its cycle performance relatively poor.

[0016] Preferably, the organic ligand in step (1) includes gallic acid, and the magnesium source includes magnesium chloride.

[0017] Preferably, the reaction temperature in step (1) is 100-140℃, for example, it can be 100℃, 105℃, 110℃, 115℃, 120℃, 125℃, 130℃, 135℃ or 140℃, etc.

[0018] Preferably, the reaction time in step (1) is 20-28 hours, for example, 20 hours, 21 hours, 22 hours, 23 hours, 24 hours, 25 hours, 26 hours, 27 hours or 28 hours.

[0019] Preferably, the mass fraction of the magnesium-based MOF is 1-5%, based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%, for example, it can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5% or 5%, etc.

[0020] In this invention, if the mass fraction of magnesium-based MOF is too low, it will result in less MOF coating and less loss of specific capacity, but poorer cycle and rate performance; if the mass fraction of magnesium-based MOF is too high, it will result in over-coating and greater loss of specific capacity, but less impact on cycle and rate performance.

[0021] Preferably, the preparation method of carbon-coated lithium vanadium oxyphosphate in step (2) includes the following steps:

[0022] (a) A lithium source, a vanadium source, a phosphorus source, a solvent and a carbon source are mixed, and after the reaction, the reaction product is calcined to obtain an intermediate.

[0023] (b) The intermediate is mixed with an oxidant, and after the reaction, the reaction product is calcined to obtain the carbon-coated lithium vanadium oxyphosphate.

[0024] Preferably, the molar ratio of the lithium source, vanadium source and phosphorus source in step (a) is (1-1.2):1:1, for example, it can be 1:1:1, 1.05:1:1, 1.1:1:1, 1.15:1:1 or 1.2:1:1, etc.

[0025] Preferably, the carbon source in step (a) includes organic acids.

[0026] Preferably, the organic acid includes at least one selected from oxalic acid, citric acid, malic acid, and tartaric acid.

[0027] Preferably, the molar ratio of the organic acid to the vanadium source is (2.1-2.7):1, for example, it can be 2.1:1, 2.2:1, 2.3:1, 2.4:1, 2.5:1, 2.6:1 or 2.7:1, etc.

[0028] Preferably, the mixing in step (a) specifically includes the following steps:

[0029] (i) Prepare the first solution and the second solution respectively:

[0030] The lithium source, phosphorus source, and solvent are mixed to obtain the first solution;

[0031] The vanadium source and the organic acid are mixed to obtain a second solution;

[0032] (ii) Mix the first solution and the second solution.

[0033] It should be noted that the order of preparing the first solution and the second solution is not important; the first solution can be prepared first, the second solution can be prepared first, or the first solution and the second solution can be prepared simultaneously.

[0034] Preferably, the reaction in step (a) is accompanied by stirring.

[0035] Preferably, the reaction time in step (a) is 1.5-2.5 h, for example, it can be 1.5 h, 1.6 h, 1.7 h, 1.8 h, 1.9 h, 2 h, 2.1 h, 2.2 h, 2.3 h, 2.4 h or 2.5 h.

[0036] Preferably, the calcination temperature in step (a) is 700-800℃, for example, it can be 700℃, 710℃, 720℃, 730℃, 740℃, 750℃, 760℃, 770℃, 780℃, 790℃ or 800℃, etc.

[0037] Preferably, the calcination time in step (a) is 6-8 hours, for example, 6 hours, 6.5 hours, 7 hours, 7.5 hours or 8 hours.

[0038] Preferably, the calcination atmosphere in step (a) is an inert atmosphere. Exemplarily, the gases in the inert atmosphere include nitrogen and argon.

[0039] Preferably, the molar ratio of the intermediate to the oxidant in step (b) is 1:(1.5-2.1), for example, it can be 1:1.5, 1:1.6, 1:1.7, 1:1.8, 1:1.9, 1:2 or 1:2.1, etc.

[0040] Preferably, the reaction temperature in step (b) is 180-220°C, for example, it can be 180°C, 185°C, 190°C, 195°C, 200°C, 205°C, 210°C or 220°C.

[0041] Preferably, the reaction time in step (b) is 10-15 hours, for example, 10 hours, 10.5 hours, 11 hours, 11.5 hours, 12 hours, 12.5 hours, 13 hours, 13.5 hours, 14 hours, 14.5 hours, or 15 hours.

[0042] Preferably, the calcination temperature in step (b) is 160-200℃, for example, it can be 160℃, 165℃, 170℃, 175℃, 180℃, 185℃, 190℃, 195℃ or 200℃, etc.

[0043] Preferably, the calcination time in step (b) is 3-6 hours, for example, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours.

[0044] Preferably, the calcination atmosphere in step (b) is an oxygen-containing atmosphere. Exemplarily, the gas in the oxygen-containing atmosphere includes air.

[0045] Preferably, the mixing in step (2) is a stepwise mixing, which includes: first mixing the carbon-coated lithium vanadium oxyphosphate with the solvent, and then adding magnesium-based MOF for a second mixing.

[0046] Preferably, the secondary mixing time is 2-3 hours, for example, 2.1 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, or 3 hours.

[0047] Preferably, in step (2), between the mixing and the sintering, a solid-liquid separation and drying step is also performed.

[0048] Preferably, the sintering temperature in step (2) is 500-600℃, for example, it can be 500℃, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 580℃, 590℃ or 600℃, etc.

[0049] In this invention, when the sintering temperature is too low, the grain boundaries between particles will be indistinct, the crystallinity of the material will be poor, the discharge specific capacity will be low, and the cycle performance will be poor. When the sintering temperature is too high, the particles will be too fluffy, the uniformity of the particles will be poor, and the discharge specific capacity will be low.

[0050] Preferably, the sintering time in step (2) is 2-4 hours, for example, it can be 2.1 hours, 2.2 hours, 2.4 hours, 2.6 hours, 2.8 hours, 3 hours, 3.1 hours, 3.2 hours, 3.4 hours, 3.6 hours, 3.8 hours or 4 hours.

[0051] Preferably, the sintering atmosphere in step (2) is an inert atmosphere. Exemplarily, the gas in the inert atmosphere includes any one of nitrogen, argon, or helium.

[0052] As a preferred technical solution of the present invention, the preparation method includes the following steps:

[0053] (I) The organic ligand, magnesium source and solvent are mixed and reacted at 100-140℃ for 20-28h to obtain magnesium-based MOF;

[0054] (III) The carbon-coated lithium vanadium oxyphosphate, solvent and the magnesium-based MOF are mixed, the solid and liquid are separated and dried, and then sintered at 500-600℃ for 2-4 hours to obtain the cathode material;

[0055] The molar ratio of the organic ligand to the magnesium source is (1.8-2.3):1; and the mass fraction of the magnesium-based MOF is 1-5%, based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%.

[0056] In a second aspect, the present invention provides a cathode material, which is prepared by the preparation method described in the first aspect.

[0057] Preferably, the cathode material comprises a lithium vanadium oxyphosphate core, and a carbon layer and a magnesium-based MOF layer sequentially coating the surface of the core from the inside out.

[0058] Thirdly, the present invention provides a lithium-ion battery, wherein the positive electrode of the lithium-ion battery includes the positive electrode material described in the second aspect.

[0059] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0060] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0061] This invention provides a method for preparing a cathode material by sequentially coating a carbon layer and a magnesium-based MOF layer onto the surface of lithium vanadium phosphate. This effectively reduces the contact between the cathode material and the electrolyte, suppresses side reactions with the electrolyte, stabilizes the material structure, and improves the cycle stability of the material. Furthermore, it can reduce the solid-solid interface resistance between the materials and the solid-liquid interface resistance between the material and the electrolyte, and improve the surface porosity, ionic conductivity, electronic conductivity, and Li-2O3 content of the material. + The insertion and extraction speed is increased, thereby improving the polarization phenomenon at high magnification and effectively enhancing the rate performance of the material. Attached Figure Description

[0062] Figure 1 shows the rate performance of batteries assembled with the cathode materials of Examples 1, 4-8 and Comparative Examples 1-4. Detailed Implementation

[0063] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0064] Example 1

[0065] This embodiment provides a method for preparing a cathode material, the method comprising the following steps:

[0066] (1) Weigh CH3COOLi (Li source) and NH4H2PO4 (P source) in a 1:1 molar ratio and dissolve them in deionized water. Stir continuously to obtain the first solution and set aside. At room temperature, weigh a certain amount of V2O5 (V source) so that the molar ratio of CH3COOLi to V2O5 is 1:1. Dissolve V2O5 in H2C2O4 aqueous solution and stir at 75℃ to obtain a light blue solution. Add citric acid so that the molar ratio of citric acid to V2O5 is 2.05:1, so that the molar ratio of organic acid to vanadium source is 2.1:1 to obtain the second solution. Mix the first solution and the second solution and stir for 2 hours. Let stand until the solution is clear.

[0067] (2) The precipitate from step 1 was placed in a tube furnace and heated from room temperature to 750°C at a rate of 5°C / min under an Ar / N2 atmosphere. The temperature was kept constant for 7 hours. After cooling to room temperature, the product was ground to obtain a black intermediate. An appropriate amount of the intermediate was dispersed in a mixed solution of ethanol and deionized water in a volume ratio of 1:1. The mixture was ultrasonically dispersed and an appropriate amount of 50% hydrogen peroxide was added as an oxidant to make the molar ratio of intermediate to oxidant 1:1.8. The mixture was reacted in a reactor at 200°C for 12 hours. After that, it was rotary evaporated, vacuum dried at 80°C, and then calcined in air at 180°C for 4 hours. After sieving, carbon-coated lithium vanadium oxyphosphate (LiVOPO4@CC) was obtained.

[0068] (3) Take 50 mL of distilled water, add 1 g of gallic acid and MgCl2, wherein the molar ratio of gallic acid to MgCl2 is 2.15:1, sonicate for 15 min, reflux at 120 °C for 24 h, cool to room temperature and filter, collect the precipitate and wash it three times with deionized water and anhydrous ethanol, sonicate, centrifuge at high speed to obtain magnesium-based MOF material (Mg-MOF);

[0069] (4) After the carbon-coated lithium vanadium oxyphosphate is ultrasonically dispersed in a solvent of deionized water and ethanol in a volume ratio of 1:1, magnesium-based MOF material is added. The mass fraction of the magnesium-based MOF is 3% based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%. The dispersion is continued for 30 min, then kept at 60°C and stirred vigorously for 2 h. The mixture is then filtered and washed several times with water and ethanol. After vacuum drying, the powder is calcined in an inert gas atmosphere at 550°C for 3 h. After cooling, the powder is sieved to obtain the cathode material (LiVOPO4@CC@Mg-MOF).

[0070] The lithium acetate, ammonium dihydrogen phosphate, magnesium chloride, vanadium pentoxide, citric acid, and gallic acid used all had a purity greater than 99%; anhydrous ethanol was of analytical grade.

[0071] Example 2

[0072] This embodiment provides a method for preparing a cathode material, the method comprising the following steps:

[0073] (1) Weigh CH3COOLi (Li source) and NH4H2PO4 (P source) in a 1:1 molar ratio and dissolve them in deionized water. Stir continuously to obtain the first solution and set aside. At room temperature, weigh a certain amount of V2O5 (V source) so that the molar ratio of CH3COOLi to V2O5 is 1:1. Dissolve V2O5 in H2C2O4 aqueous solution and stir at 75℃ to obtain a light blue solution. Add citric acid so that the molar ratio of citric acid to V2O5 is 2.05:1, so that the molar ratio of organic acid to vanadium source is 2.1:1 to obtain the second solution. Mix the first solution and the second solution and stir for 2 hours. Let stand until the solution is clear.

[0074] (2) The precipitate from step 1 was placed in a tube furnace and heated from room temperature to 700°C at a rate of 5°C / min under an Ar / N2 atmosphere. The temperature was kept constant for 8 hours. After cooling to room temperature, it was ground to obtain a black intermediate. An appropriate amount of the intermediate was dispersed in a mixed solution of ethanol and deionized water in a volume ratio of 1:1. The mixture was ultrasonically dispersed and an appropriate amount of 50% hydrogen peroxide was added as an oxidant to make the molar ratio of intermediate to oxidant 1:2.1. The mixture was reacted in a reactor at 180°C for 15 hours. After that, it was rotary evaporated, vacuum dried at 80°C, and then calcined in air at 200°C for 3 hours. After sieving, carbon-coated lithium vanadium oxyphosphate (LiVOPO4@CC) was obtained.

[0075] (3) Take 50 mL of distilled water, add 1 g of gallic acid and MgCl2, wherein the molar ratio of gallic acid to MgCl2 is 1.8:1, sonicate for 15 min, reflux at 100 °C for 28 h, cool to room temperature and filter, collect the precipitate and wash it three times with deionized water and anhydrous ethanol, sonicate, centrifuge at high speed to obtain magnesium-based MOF material (Mg-MOF);

[0076] (4) After the carbon-coated lithium vanadium oxyphosphate is ultrasonically dispersed in a solvent of deionized water and ethanol in a volume ratio of 1:1, magnesium-based MOF material is added. The mass fraction of magnesium-based MOF is 1% based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%. The dispersion is continued for 30 min, then kept at 60°C and stirred vigorously for 2 h. After filtration, the powder is washed several times with water and ethanol, dried under vacuum, and then calcined in an inert gas atmosphere at 500°C for 4 h. After cooling, the powder is sieved to obtain the cathode material (LiVOPO4@CC@Mg-MOF).

[0077] The lithium acetate, ammonium dihydrogen phosphate, magnesium chloride, vanadium pentoxide, citric acid, and gallic acid used all had a purity greater than 99%; anhydrous ethanol was of analytical grade.

[0078] Example 3

[0079] This embodiment provides a method for preparing a cathode material, the method comprising the following steps:

[0080] (1) Weigh CH3COOLi (Li source) and NH4H2PO4 (P source) in a 1:1 molar ratio and dissolve them in deionized water. Stir continuously to obtain the first solution and set aside. At room temperature, weigh a certain amount of V2O5 (V source) so that the molar ratio of CH3COOLi to V2O5 is 1:1. Dissolve V2O5 in H2C2O4 aqueous solution and stir at 75℃ to obtain a light blue solution. Add citric acid so that the molar ratio of citric acid to V2O5 is 2.65:1, so that the molar ratio of organic acid to vanadium source is 2.7:1 to obtain the second solution. Mix the first solution and the second solution and stir for 2 hours. Let stand until the solution is clear.

[0081] (2) The precipitate from step 1 was placed in a tube furnace and heated from room temperature to 800°C at a rate of 5°C / min under an Ar / N2 atmosphere. The temperature was kept constant for 6 hours. After cooling to room temperature, it was ground to obtain a black intermediate. An appropriate amount of the intermediate was dispersed in a mixed solution of ethanol and deionized water in a volume ratio of 1:1. The mixture was ultrasonically dispersed and an appropriate amount of 50% hydrogen peroxide was added as an oxidant to make the molar ratio of intermediate to oxidant 1:1.5. The mixture was reacted in a reactor at 220°C for 10 hours. After that, it was rotary evaporated, vacuum dried at 80°C, and then calcined in air at 160°C for 6 hours. After sieving, carbon-coated lithium vanadium oxyphosphate (LiVOPO4@CC) was obtained.

[0082] (3) Take 50 mL of distilled water, add 1 g of gallic acid and MgCl2, wherein the molar ratio of gallic acid to MgCl2 is 2.3:1, sonicate for 15 min, reflux at 140 °C for 20 h, cool to room temperature and filter, collect the precipitate and wash it three times with deionized water and anhydrous ethanol, sonicate, centrifuge at high speed to obtain magnesium-based MOF material (Mg-MOF);

[0083] (4) After the carbon-coated lithium vanadium oxyphosphate is ultrasonically dispersed in a solvent of deionized water and ethanol in a volume ratio of 1:1, magnesium-based MOF material is added. The mass fraction of the magnesium-based MOF is 5% based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%. The dispersion is continued for 30 min, then kept at 60°C and stirred vigorously for 2 h. The mixture is then filtered and washed several times with water and ethanol. After vacuum drying, the powder is calcined in an inert gas atmosphere at 600°C for 2 h. After cooling, the powder is sieved to obtain the cathode material (LiVOPO4@CC@Mg-MOF).

[0084] The lithium acetate, ammonium dihydrogen phosphate, magnesium chloride, vanadium pentoxide, citric acid, and gallic acid used all had a purity greater than 99%; anhydrous ethanol was of analytical grade.

[0085] Example 4

[0086] The difference between this embodiment and embodiment 1 is that in step (4), the mass fraction of magnesium-based MOF is adjusted to 0.5%, while the rest is exactly the same as in embodiment 1.

[0087] Example 5

[0088] The difference between this embodiment and Embodiment 1 is that in step (4), the mass fraction of magnesium-based MOF is adjusted to 6%, while the rest is exactly the same as in Embodiment 1.

[0089] Example 6

[0090] The difference between this embodiment and embodiment 1 is that in step (4), the sintering temperature is adjusted to 450°C, while the rest is exactly the same as in embodiment 1.

[0091] Example 7

[0092] The difference between this embodiment and embodiment 1 is that in step (4), the sintering temperature is adjusted to 650°C, while the rest is exactly the same as in embodiment 1.

[0093] Example 8

[0094] The difference between this embodiment and embodiment 1 is that in step (1), the aqueous solutions of CH3COOLi, NH4H2PO4, V2O5, and H2C2O4 and citric acid are directly mixed instead of being mixed in steps. The rest is exactly the same as in embodiment 1.

[0095] Comparative Example 1

[0096] The difference between this comparative example and Example 1 is that steps (3) and (4) are omitted, that is, magnesium-based MOF coating is not performed. Otherwise, it is exactly the same as Example 1.

[0097] Comparative Example 2

[0098] The difference between this comparative example and Example 1 is that the carbon-coated lithium vanadium phosphate is replaced with lithium vanadium phosphate, that is, the surface of the lithium vanadium phosphate is not coated with a carbon layer. Otherwise, it is exactly the same as Example 1.

[0099] Comparative Example 3

[0100] The difference between this comparative example and Example 1 is that the molar ratio of gallic acid to magnesium chloride is adjusted to 1.7:1, while the rest is exactly the same as Example 1.

[0101] Comparative Example 4

[0102] The difference between this comparative example and Example 1 is that the molar ratio of gallic acid to magnesium chloride is adjusted to 2.4:1, while the rest is exactly the same as Example 1.

[0103] Performance testing

[0104] The positive electrode material, acetylene black, and polyvinylidene fluoride (PVDF) provided in Examples 1-8 and Comparative Examples 1-4 were used to prepare positive electrode sheets in a mass ratio of 75:15:10; the negative electrode sheet was a pure lithium sheet; the positive electrode sheet, separator, negative electrode sheet, and nickel foil were assembled into a 2025 coin cell. The positive and negative electrodes of the battery were made of 2025 stainless steel, and the electrolyte was 1 mol / L LiPF6 / EC+DEC, where EC is ethylene carbonate, DEC is diethyl carbonate, and the volume ratio of EC to DEC was 1:1; the battery assembly was carried out in a glove box under a N2 / Ar atmosphere.

[0105] (1) Cyclic performance test

[0106] Test conditions: The battery was charged and discharged at a rate of 0.05C for a total of 50 cycles, where 1C = 159mAh·g -1 The voltage range is 2.5V-4.5V (vs. Li). + / Li), the test temperature is 25±3℃; the first discharge specific capacity, the discharge specific capacity of the battery after 50 cycles and the capacity retention rate after 50 cycles are obtained;

[0107] The test results of Examples 1-8 and Comparative Examples 1-4 are shown in Table 1.

[0108] Table 1

[0109]

[0110]

[0111] (2) Ratio Performance Test

[0112] Test conditions: The battery was charged and discharged sequentially at rates of 0.05C, 0.1C, 0.2C, 0.5C, and 0.05C, with 10 charge-discharge cycles performed at each rate. 1C = 159 mAh·g -1 The voltage range is 2.5V-4.5V (vs. Li). + / Li), the test temperature is 25±3℃; obtain the discharge specific capacity of the battery per cycle at all rates;

[0113] The test results of Examples 1, 4-8 and Comparative Examples 1-4 are summarized in Figure 1. The horizontal axis of the figure is the number of cycles, and the vertical axis is the discharge specific capacity, with the unit being mAh / g.

[0114] analyze:

[0115] As shown in Table 1 and Figure 1, the preparation method of this invention, which sequentially coats the surface of lithium vanadium oxyphosphate with a carbon layer and a magnesium-based MOF layer, effectively reduces the contact between the cathode material and the electrolyte, suppresses side reactions with the electrolyte, stabilizes the material structure, improves the cycle stability of the material, and reduces the solid-solid interface resistance between the materials and the solid-liquid interface resistance between the material and the electrolyte, thereby increasing the surface porosity, ionic conductivity, electronic conductivity, and Li... + This method improves the insertion / extraction rate, thereby reducing polarization at high rates and effectively enhancing the rate performance of the material. The cathode material prepared using this method produces batteries that exhibit excellent cycle performance and rate performance.

[0116] The results from Examples 1, 4, and 5 show that if the mass fraction of magnesium-based MOF is too low, the MOF coating will be less, resulting in less loss of specific capacity of the lithium vanadium phosphate main material, but poor cycle and rate performance. If the mass fraction of magnesium-based MOF is too high, it will lead to over-coating, resulting in greater loss of specific capacity of the material, but less impact on cycle and rate performance.

[0117] The results of Examples 1, 6 and 7 show that when the sintering temperature is too low, the grain boundaries between particles are not obvious, the crystallinity of the material is poor, the initial discharge specific capacity is low, and the cycle performance is poor. When the sintering temperature is too high, the particles are too loose, the uniformity of the particles is poor, and the discharge specific capacity is low.

[0118] The results of Examples 1 and 8 show that when raw materials are blended, the particles become uneven, the crystal form is not fixed and the particle size distribution is uneven, the stability of the material is also poor, resulting in a low discharge specific capacity of the material, and a decrease in rate and cycle performance.

[0119] The results of Example 1, Comparative Example 1 and Comparative Example 2 show that when the magnesium-based MOF is not coated, the discharge specific capacity is high, but the conductivity of the main material is poor. Since the MOF is not coated, the contact with the electrolyte cannot be reduced, the side reactions increase, and the cycle performance decreases. When the carbon layer is not coated, the conductivity of the material is poor, the discharge specific capacity is low, there are more side reactions between the electrolyte and the main material, and the rate capability and cycle performance are poor.

[0120] The results of Examples 1, 3, and 4 show that when the molar ratio of organic ligand to magnesium source is too low, the synthesized MOF will be less, that is, the coating of the main material will be less, the side reaction between the electrolyte and the main material will increase, and the cycle performance will be poor. When the molar ratio of organic ligand to magnesium source is too high, the impurity phase will be formed, and the side reaction with the electrolyte will make its cycle performance relatively poor.

[0121] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a positive electrode material, characterized in that, The preparation method includes: (1) mixing an organic ligand, a magnesium source, and a solvent, and reacting to obtain a magnesium-based MOF; wherein the molar ratio of the organic ligand to the magnesium source is (1.8-2.3):1; the organic ligand includes gallic acid, and the magnesium source includes magnesium chloride; (2) mixing carbon-coated lithium vanadium oxyphosphate, a solvent, and the magnesium-based MOF, and sintering at a temperature of 500-590°C to obtain the cathode material; based on the mass of the carbon-coated lithium vanadium oxyphosphate being 100%, the mass fraction of the magnesium-based MOF is 1-5%; the carbon-coated lithium vanadium oxyphosphate... The method for preparing lithium vanadium includes the following steps: (a) (i) preparing a first solution and a second solution respectively: mixing a lithium source, a phosphorus source and a solvent to obtain a first solution; mixing a vanadium source and an organic acid to obtain a second solution; (ii) mixing the first solution and the second solution, reacting, and then calcining the reaction product to obtain an intermediate; (b) mixing the intermediate with an oxidant, reacting, and then calcining the reaction product to obtain the carbon-coated lithium vanadium oxyphosphate; the organic acid in step (a) includes at least one of oxalic acid, citric acid, malic acid and tartaric acid.

2. The preparation method according to claim 1, characterized in that, The reaction temperature in step (1) is 100-140℃.

3. The preparation method according to claim 1, characterized in that, The reaction time in step (1) is 20-28 hours.

4. The preparation method according to claim 1, characterized in that, The molar ratio of the lithium source, vanadium source and phosphorus source in step (a) is (1-1.2):1:

1.

5. The preparation method according to claim 1, characterized in that, The molar ratio of the organic acid to the vanadium source is (2.1-2.7):

1.

6. The preparation method according to claim 1, characterized in that, The reaction described in step (a) is accompanied by stirring.

7. The preparation method according to claim 1, characterized in that, The reaction time in step (a) is 1.5-2.5 h.

8. The preparation method according to claim 1, characterized in that, The calcination temperature in step (a) is 700-800℃.

9. The preparation method according to claim 1, characterized in that, The calcination time in step (a) is 6-8 hours.

10. The preparation method according to claim 1, characterized in that, The calcination atmosphere in step (a) is an inert atmosphere.

11. The preparation method according to claim 1, characterized in that, The oxidant in step (b) includes hydrogen peroxide.

12. The preparation method according to claim 1, characterized in that, The molar ratio of the intermediate to the oxidant in step (b) is 1:(1.5-2.1).

13. The preparation method according to claim 1, characterized in that, The reaction temperature in step (b) is 180-220℃.

14. The preparation method according to claim 1, characterized in that, The reaction time in step (b) is 10-15 hours.

15. The preparation method according to claim 1, characterized in that, The calcination temperature in step (b) is 160-200℃.

16. The preparation method according to claim 1, characterized in that, The calcination time in step (b) is 3-6 hours.

17. The preparation method according to claim 1, characterized in that, The calcination atmosphere in step (b) is an oxygen-containing atmosphere.

18. The preparation method according to claim 1, characterized in that, The mixing in step (2) is a step-by-step mixing, which includes: first mixing the carbon-coated lithium vanadium oxyphosphate with the solvent, and then adding magnesium-based MOF for a second mixing.

19. The preparation method according to claim 18, characterized in that, The secondary mixing time is 2-3 hours.

20. The preparation method according to claim 1, characterized in that, In step (2), between the mixing and the sintering, a solid-liquid separation and drying step is also performed.

21. The preparation method according to claim 1, characterized in that, The sintering time in step (2) is 2-4 hours.

22. The preparation method according to claim 1, characterized in that, The sintering atmosphere in step (2) is an inert atmosphere.

23. The preparation method according to claim 1, characterized in that, The preparation method includes the following steps: (I) mixing an organic ligand, a magnesium source, and a solvent, reacting at 100-140℃ for 20-28 h to obtain a magnesium-based MOF; (III) mixing carbon-coated lithium vanadium oxyphosphate, a solvent, and the magnesium-based MOF, separating the solid and liquid phases, drying the mixture, and then sintering it at 500-590℃ for 2-4 h to obtain the cathode material; wherein, the molar ratio of the organic ligand to the magnesium source is (1.8-2.3):1; the organic ligand includes gallic acid, and the magnesium source includes magnesium chloride; based on the mass of the carbon-coated lithium vanadium oxyphosphate as 100%, the mass fraction of the magnesium-based MOF is... The number is 1-5%; the preparation method of the carbon-coated lithium vanadium oxyphosphate includes the following steps: (a) (i) preparing a first solution and a second solution respectively: mixing a lithium source, a phosphorus source and a solvent to obtain a first solution; mixing a vanadium source and an organic acid to obtain a second solution; (ii) mixing the first solution and the second solution, reacting, and calcining the reaction product to obtain an intermediate; (b) mixing the intermediate with an oxidant, reacting, and calcining the reaction product to obtain the carbon-coated lithium vanadium oxyphosphate; the organic acid in step (a) includes at least one of oxalic acid, citric acid, malic acid and tartaric acid.

24. A positive electrode material, characterized in that, The cathode material is prepared by the preparation method according to any one of claims 1-23.

25. The cathode material according to claim 24, characterized in that, The cathode material comprises a lithium vanadium oxyphosphate core, and a carbon layer and a magnesium-based MOF layer sequentially coating the surface of the core from the inside out.

26. A lithium-ion battery, characterized in that, The positive electrode of the lithium-ion battery includes the positive electrode material as described in claim 24 or 25.

Images