A graphene-coated lithium iron phosphate cathode material and its preparation method

Abstract

The present invention discloses a graphene-coated lithium iron phosphate cathode material and a preparation method thereof, belonging to the technical field of lithium iron phosphate cathode materials. First, iron phosphate is loaded on graphene, and by controlling the pH adjustment time, small-particle-size FePO 4 / GO and large-particle-size FePO 4 / GO are accurately obtained. Iron phosphate is loaded on single-walled carbon nanotubes to prepare FePO 4 / SWCNT. Then, the loaded material and a lithium source are subjected to a pre-calcination reaction to grow and form graphene-coated lithium iron phosphate. Finally, dopamine is oxidized and self-polymerized to form polydopamine, which is coated on the outer layer of the graphene-coated lithium iron phosphate. After calcination and carbonization, a double coating of porous carbon material and graphene is formed. The particle size distribution of the prepared cathode material is optimized by using iron phosphate materials with different particle sizes and shapes to improve its tap density, and the conductivity is improved by graphene coating, endowing the cathode material with excellent electrochemical performance.

Description

Technical Field

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

[0002] Lithium iron phosphate LiFePO 4 (LFP) occupies a place in the cathode material market due to its high theoretical capacity, high safety performance and high structural stability. However, the LiFePO 4 material has inherent disadvantages of low electronic conductivity (10 -9 to 10 -8 S·cm -1 ) and low ionic conductivity (10 -11 to 10 -9 S·cm -1 ), which will cause high impedance and polarization problems during charge and discharge processes, as well as the subsequent problems of poor actual specific capacity and rate performance. Therefore, it is necessary to improve the kinetic performance of the LiFePO 4 material, thereby improving the capacity of the material at different charge and discharge rates.

[0003] Secondly, in order to meet the market's requirements for higher energy density of lithium-ion batteries, on the premise of unchanged volume, to increase the energy density of the battery, it is necessary to increase the compaction density of the LiFePO 4 cathode sheet. The compaction density of the LiFePO 4 cathode sheet in most existing commercial battery cells is 2.4 - 2.5 g / cm 3 , and it is necessary to further increase the compaction density to meet higher requirements for the energy density of the battery. Summary of the Invention

[0004] The purpose of the present invention is to provide a graphene-coated lithium iron phosphate cathode material and a preparation method thereof, so as to improve the compaction density and electrochemical performance of the cathode material.

[0005] The purpose of the present invention can be achieved by the following technical solutions:

[0006] The present invention provides a preparation method of a graphene-coated lithium iron phosphate cathode material, comprising the following steps:

[0007] Step 1: Mix small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO and FePO 4 / SWCNT in a mass ratio of (3 - 5):(1 - 2):(0.5 - 1) to form a mixture;

[0008] Step 2: Add a lithium source and water to the mixture to form a suspension, and spray-dry to obtain a solid material. Pre-calcine the solid material at 700 - 750 °C for 6 - 8 h under a nitrogen atmosphere to form a precursor.

[0009] Step 3: Disperse the precursor in water at a mass concentration of 1 - 3 g / L, add hydrochloric acid dopamine and ammonia water, react at 30 - 40 °C for 6 - 8 h, filter and then dry under vacuum, and calcine at 700 - 750 °C for 2 - 4 h under a nitrogen atmosphere to obtain the cathode material.

[0010] Further, the small-particle-size FePO 4 / GO and the large-particle-size FePO 4 / GO are both iron phosphate precipitated and loaded on graphene oxide, and FePO 4 / SWCNT is iron phosphate precipitated and loaded on single-walled carbon nanotubes;

[0011] The particle size of the small-particle-size FePO 4 / GO is 100 - 200 nm, and the particle size of the large-particle-size FePO 4 / GO is 3 - 5 μm.

[0012] Further, the preparation steps of the small-particle-size FePO 4 / GO are as follows:

[0013] After mixing the ferrous sulfate solution and the graphene oxide dispersion liquid, add phosphoric acid solution according to the molar ratio of iron to phosphorus of 1:1.05 - 1.08. At 40 - 50 °C, add 20 - 30 wt% H 2 O 2 dropwise according to the molar ratio of H 2 O 2 to divalent iron ions of 0.5 - 0.6:1, stir for the oxidation reaction for 1 - 2 h, add 20 - 30 wt% ammonia water dropwise to adjust the pH to 1.5 - 2.0, the dropping time is 10 - 20 min, filter, wash and dry to obtain small-particle-size FePO 4 / GO;

[0014] The preparation steps of the large-particle-size FePO 4 / GO are as follows:

[0015] After mixing the ferrous sulfate solution and the graphene oxide dispersion liquid, add phosphoric acid solution according to the molar ratio of iron to phosphorus of 1:1.05 - 1.08. At 40 - 50 °C, add 20 - 30 wt% H 2 O 2 dropwise according to the molar ratio of H 2 O 2The solution was stirred for the oxidation reaction for 1 - 2 h, and 20 - 30 wt% ammonia water was added dropwise to adjust the pH to 1.5 - 2.0. The dropping time was 50 - 60 min. After filtration, washing, and drying, large - particle - size FePO 4 / GO was obtained.

[0016] The phosphoric acid inserted between the graphene oxide sheets weakens the interaction between the sheets, avoids the reduction of reaction sites caused by particle agglomeration, and improves the reaction activity. The phosphoric acid molecule has strong polarity. After being introduced into GO, it will form hydrogen bonds with the oxygen - containing functional groups (such as carboxyl groups, hydroxyl groups, etc.) on the surface of GO. This hydrogen - bond interaction can reduce the van der Waals force and π - π stacking interaction between the GO sheets, thereby weakening the mutual attraction between the sheets. The phosphoric acid molecules are inserted into the interlayer of GO, increasing the interlayer distance, thus reducing the packing density between the sheets and facilitating the loading of iron phosphate between the layers.

[0017] Large - particle - size FePO 4 / GO is formed by extending the dropping time of ammonia water, enabling the newly formed particles to attach to the initially formed particles for secondary growth. The particle size gradually increases, and the primary particles aggregate to form secondary particles.

[0018] In the preparation of lithium iron phosphate of the present invention, iron phosphate is first loaded on graphene. Through the high specific surface area of graphene, iron phosphate is uniformly dispersed on the surface and between the layers of graphene, effectively reducing the agglomeration of iron phosphate particles, controlling the particle size of iron phosphate, and precisely obtaining small - particle - size FePO 4 / GO and large - particle - size FePO 4 / GO. Then, the loaded material is subjected to a pre - calcination reaction with a lithium source to grow into lithium iron phosphate coated with graphene. Finally, polydopamine is formed by the oxidative self - polymerization of dopamine and coated on the outer layer of the graphene - coated lithium iron phosphate. After calcination and carbonization, a double - layer coating of porous carbon material and graphene is formed, and the conductivity of the cathode material is significantly improved.

[0019] Furthermore, the concentration of the ferrous sulfate solution is 1 - 2 mol / L, the concentration of the graphene oxide dispersion is 4 - 5 g / L, and the mass ratio of the ferrous sulfate solution to the graphene oxide dispersion is 100:3 - 4 in terms of ferrous sulfate and graphene oxide.

[0020] Furthermore, the preparation steps of the FePO 4 / SWCNT are as follows:

[0021] After the ferrous sulfate solution and the single - walled carbon nanotube dispersion are mixed, phosphoric acid solution is added according to the molar ratio of iron to phosphorus of 1:1.05 - 1.08. At 40 - 50 °C, according to the molar ratio of H 2 O 2 and divalent iron ions of 0.5 - 0.6:1, 20 - 30 wt% H2 O 2 solution, stir the oxidation reaction for 1 - 2 h, add 20 - 30 wt% ammonia water to adjust the pH to 1.5 - 2.0, the addition time is 10 - 20 min, filter, wash and dry to obtain FePO 4 / SWCNT.

[0022] Single - walled carbon nanotubes are a type of graphene material and are made of single - layer graphene. Loading iron phosphate on single - walled carbon nanotubes can promote the dispersion of iron phosphate. At the same time, its high aspect - ratio structure can construct a three - dimensional nanostructure during the co - calcination with the graphene - loaded iron phosphate material, prevent the agglomeration of the graphene - coated material, and optimize the particle size distribution of the cathode material.

[0023] Further, the length of the single - walled carbon nanotubes is 1 - 2 μm, and the diameter is 1 - 2 nm.

[0024] Further, the concentration of the ferrous sulfate solution is 1 - 2 mol / L, the concentration of the single - walled carbon nanotube dispersion is 1 - 2 g / L, and the mass ratio of the ferrous sulfate solution to the single - walled carbon nanotube dispersion is 100:3 - 4 in terms of the mass of ferrous sulfate and single - walled carbon nanotubes.

[0025] Further, the lithium source is one of lithium carbonate and lithium hydroxide.

[0026] Further, the mixture and the lithium source in the suspension are mixed at a molar ratio of iron to lithium of 1:1 - 1.02, and the mass fraction of water in the suspension is 50 - 60%.

[0027] Further, the mass ratio of dopamine hydrochloride to the precursor is 2 - 5:100;

[0028] The ammonia water adjusts the pH of the reaction system to 8.2 - 8.5.

[0029] The present invention also provides a graphene - coated lithium iron phosphate cathode material prepared by the above - described preparation method.

[0030] Advantages of the present invention:

[0031] (1) In the present invention, the cathode material is prepared by mixing small - particle - size FePO 4 / GO, large - particle - size FePO 4 / GO and FePO 4 / SWCNT. The small - particle - size particles can effectively fill the voids between the large - particle - size particles, thereby reducing the porosity of the material. The FePO 4 / SWCNT cylinders and the spherical or quasi - spherical FePO 4 / GO materials are interlaced and combined, and the mutual filling effect can form a continuous and dense cathode material, endowing the cathode material with a high tap density.

[0032] (2) In the present invention, iron phosphate is precipitated and loaded on carbon nanotubes and graphene, which can promote the dispersion of iron phosphate particles, avoid excessive particle size caused by aggregation, and by extending the time for adjusting the pH, reduce the initial nucleation particles, induce the formation of secondary particles, and form small-sized FePO with different particle sizes 4 / GO and large-sized FePO 4 / GO, and then react with the lithium source to generate lithium iron phosphate materials with different particle sizes, optimizing the particle size distribution of the cathode material. Moreover, carbon nanotubes and graphene will form a coating on lithium iron phosphate during the subsequent calcination process, improving its conductivity.

[0033] (3) In the present invention, the precursor and dopamine are mixed, and poly-dopamine coating is formed on the surface of the precursor material through the oxidative self-polymerization of dopamine. After calcination, it is carbonized to form an outer carbon-coated material, forming a double-layer coating on the cathode material to avoid the exposure of active components to the electrolyte. The porous amorphous carbon layer after the calcination of poly-dopamine can serve as a buffer layer to reduce the volume expansion or shedding of graphene during charge and discharge, improving the cycle stability of the material. Detailed implementation manners

[0034] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0035] Example 1

[0036] Preparation of small-sized FePO 4 / GO:

[0037] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. Mix the ferrous sulfate solution and the graphene oxide dispersion according to the mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to the molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. According to the molar ratio of H 2 O 2 and divalent iron ions of 0.5:1, add 30 wt% H 2 O 2The solution was stirred for the oxidation reaction for 2 h, 20 wt% ammonia water was added dropwise to adjust the pH to 1.7, the dropping time was 10 min, and after filtration, washing and drying, small-sized FePO 4 / GO with a particle size of 100 - 150 nm was obtained.

[0038] Preparation of large-sized FePO 4 / GO:

[0039] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion solution with a concentration of 5 g / L. Mix the ferrous sulfate solution and the graphene oxide dispersion solution according to the mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to the molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Add 30 wt% H 2 O 2 dropwise according to the molar ratio of H 2 O 2 to divalent iron ions of 0.5:1. Stir the oxidation reaction for 2 h, add 20 wt% ammonia water dropwise to adjust the pH to 1.7, the dropping time is 50 min, and after filtration, washing and drying, large-sized FePO 4 / GO with a particle size of 3 - 4 μm was obtained.

[0040] Preparation of FePO 4 / SWCNT:

[0041] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh single-walled carbon nanotubes (length: 1 - 2 μm, diameter: 1 - 2 nm) and add them to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a single-walled carbon nanotube dispersion solution with a concentration of 2 g / L. Mix the ferrous sulfate solution and the single-walled carbon nanotube dispersion solution according to the mass ratio of ferrous sulfate to single-walled carbon nanotubes of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to the molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Add 30 wt% H 2 O 2 dropwise according to the molar ratio of H 2 O 2 to divalent iron ions of 0.5:1. Stir the oxidation reaction for 2 h, add 20 wt% ammonia water dropwise to adjust the pH to 1.7, the dropping time is 10 min, and after filtration, washing and drying, FePO was obtained.4 / SWCNT.

[0042] Preparation of the positive electrode material:

[0043] Step 1: Mix small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT in a mass ratio of 3:2:1 to form a mixture;

[0044] Step 2: Add lithium carbonate to the mixture. The addition amount of lithium carbonate is calculated according to the mass of iron elements added during the preparation of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT. Mix with lithium carbonate in a molar ratio of iron to lithium of 1:1, then add deionized water to form a suspension. The mass fraction of water in the suspension is 60%. After spray drying, a solid material is obtained and pre-calcined at 700 °C for 8 h under a nitrogen atmosphere to form a precursor;

[0045] Step 3: Add the precursor to deionized water at a mass concentration of 1 g / L, ultrasonically disperse for 2 h, add dopamine hydrochloride in a mass ratio of dopamine hydrochloride to the precursor of 5:100, add ammonia water to adjust the pH to 8.5, control the temperature at 40 °C by water bath heating, stir and react for 6 h, filter and then dry in vacuum, and calcine at 700 °C for 4 h under a nitrogen atmosphere to obtain the positive electrode material.

[0046] Example 2

[0047] The difference from Example 1 is that the mass ratio of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT is adjusted to 5:2:1, and other conditions and steps are the same as in Example 1.

[0048] Example 3

[0049] The difference from Example 1 is that the mass ratio of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT is adjusted to 4:1.5:0.75, and other conditions and steps are the same as in Example 1.

[0050] Example 4

[0051] The difference from Example 3 is that when preparing small-particle-size FePO 4 / GO and large-particle-size FePO 4During the / GO process, the ferrous sulfate solution and the graphene oxide dispersion were mixed according to a mass ratio of ferrous sulfate to graphene oxide of 100:3.5 to prepare FePO 4 During the / SWCNT process, the ferrous sulfate solution and the single-walled carbon nanotube dispersion were mixed according to a mass ratio of ferrous sulfate to single-walled carbon nanotubes of 100:3.5, and other conditions and steps were the same as in Example 3.

[0052] Example 5

[0053] The difference from Example 3 was that small-particle-size FePO 4 / GO and large-particle-size FePO 4 During the / GO process, the ferrous sulfate solution and the graphene oxide dispersion were mixed according to a mass ratio of ferrous sulfate to graphene oxide of 100:4 to prepare FePO 4 During the / SWCNT process, the ferrous sulfate solution and the single-walled carbon nanotube dispersion were mixed according to a mass ratio of ferrous sulfate to single-walled carbon nanotubes of 100:4, and other conditions and steps were the same as in Example 3.

[0054] Example 6

[0055] The difference from Example 3 was that the mass ratio of dopamine hydrochloride to the precursor was adjusted from 5:100 to 3:100, and the specific steps were as follows:

[0056] Prepare small-particle-size FePO 4 / GO:

[0057] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion with a concentration of 5 g / L. The ferrous sulfate solution and the graphene oxide dispersion were mixed according to a mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, phosphoric acid solution (concentration: 85 wt%) was added according to a molar ratio of iron to phosphorus of 1:1.05. The temperature was controlled at 40 °C by water bath heating. H 2 O 2 and divalent iron ions were added dropwise at a molar ratio of 0.5:1 of 30 wt% H 2 O 2 solution, and the oxidation reaction was stirred for 2 h. 20 wt% ammonia water was added dropwise to adjust the pH to 1.7, and the dropping time was 10 min. After filtration, washing, and drying, small-particle-size FePO 4 / GO with a particle size of 100 - 150 nm was obtained.

[0058] Prepare large-particle-size FePO4 / GO:

[0059] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. Mix the ferrous sulfate solution and the graphene oxide dispersion according to a mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to a molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Add 30 wt% H 2 O 2 dropwise according to a molar ratio of H 2 O 2 to divalent iron ions of 0.5:1, stir and oxidize for 2 h, add 20 wt% ammonia water dropwise to adjust the pH to 1.7, with a dropping time of 50 min, filter, wash, and dry to obtain large-particle-size FePO 4 / GO, and its particle size is 3 - 4 μm.

[0060] Preparation of FePO 4 / SWCNT:

[0061] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh single-walled carbon nanotubes (length: 1 - 2 μm, diameter: 1 - 2 nm) and add them to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a single-walled carbon nanotube dispersion. The concentration of the single-walled carbon nanotube dispersion is 2 g / L. Mix the ferrous sulfate solution and the single-walled carbon nanotube dispersion according to a mass ratio of ferrous sulfate to single-walled carbon nanotubes of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to a molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Add 30 wt% H 2 O 2 dropwise according to a molar ratio of H 2 O 2 to divalent iron ions of 0.5:1, stir and oxidize for 2 h, add 20 wt% ammonia water dropwise to adjust the pH to 1.7, with a dropping time of 10 min, filter, wash, and dry to obtain FePO 4 / SWCNT.

[0062] Preparation of the positive electrode material:

[0063] Step 1. Mix small-particle-size FePO 4 / GO and large-particle-size FePO 4 / GO and FePO 4 / SWCNT are mixed in a mass ratio of 4:1.5:0.75 to form a mixture;

[0064] Step 2: Lithium carbonate is added to the mixture. The addition amount of lithium carbonate is calculated according to the mass of iron elements added during the preparation of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO and FePO 4 / SWCNT. Lithium carbonate is added and mixed according to a molar ratio of iron to lithium of 1:1, and then deionized water is added to form a suspension. The mass ratio of water in the suspension is 60%. After spray drying, a solid material is obtained and pre-calcined at 700°C for 8 h under a nitrogen atmosphere to form a precursor;

[0065] Step 3: The precursor is added to deionized water at a mass concentration of 1 g / L, ultrasonically dispersed for 2 h, dopamine hydrochloride is added according to a mass ratio of dopamine hydrochloride to the precursor of 3:100, and ammonia water is added to adjust the pH to 8.5. The temperature is controlled at 40°C by water bath heating, and the stirring reaction is carried out for 6 h. After filtration, it is dried in vacuum and calcined at 700°C for 3 h under a nitrogen atmosphere to obtain the cathode material.

[0066] Example 7

[0067] The difference from Example 3 is that the mass ratio of dopamine hydrochloride to the precursor is adjusted from 5:100 to 2:100. The specific steps are as follows:

[0068] Preparation of small-particle-size FePO 4 / GO:

[0069] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and ultrasonically disperse it for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. The ferrous sulfate solution and the graphene oxide dispersion are mixed according to a mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, phosphoric acid solution (concentration: 85 wt%) is added according to a molar ratio of iron to phosphorus of 1:1.05. The temperature is controlled at 40°C by water bath heating. H 2 O 2 and divalent iron ions are added dropwise at a molar ratio of 0.5:1 of 30 wt% H 2 O 2 solution, and the oxidation reaction is stirred for 2 h. 20 wt% ammonia water is added dropwise to adjust the pH to 1.7, and the dropping time is 10 min. After filtration, washing and drying, small-particle-size FePO 4 / GO is obtained, and its particle size is 100 - 150 nm.

[0070] Preparation of large-sized FePO 4 / GO:

[0071] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. Mix the ferrous sulfate solution and the graphene oxide dispersion according to a mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, add a phosphoric acid solution (concentration: 85 wt%) according to a molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Dropwise add 30 wt% H 2 O 2 solution according to a molar ratio of H 2 O 2 to divalent iron ions of 0.5:1, stir the oxidation reaction for 2 h, dropwise add 20 wt% ammonia water to adjust the pH to 1.7, with a dropping time of 50 min, filter, wash, and dry to obtain large-sized FePO 4 / GO, with a particle size of 3 - 4 μm.

[0072] Preparation of FePO 4 / SWCNT:

[0073] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh single-walled carbon nanotubes (length: 1 - 2 μm, diameter: 1 - 2 nm) and add them to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a single-walled carbon nanotube dispersion. The concentration of the single-walled carbon nanotube dispersion is 2 g / L. Mix the ferrous sulfate solution and the single-walled carbon nanotube dispersion according to a mass ratio of ferrous sulfate to single-walled carbon nanotubes of 100:3. After mixing, add a phosphoric acid solution (concentration: 85 wt%) according to a molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water bath heating. Dropwise add 30 wt% H 2 O 2 solution according to a molar ratio of H 2 O 2 to divalent iron ions of 0.5:1, stir the oxidation reaction for 2 h, dropwise add 20 wt% ammonia water to adjust the pH to 1.7, with a dropping time of 10 min, filter, wash, and dry to obtain FePO 4 / SWCNT.

[0074] Preparation of the positive electrode material:

[0075] Step 1. Small-sized FePO 4 / GO, large particle size FePO 4 / GO and FePO 4 / SWCNT are mixed in a mass ratio of 4:1.5:0.75 to form a mixture;

[0076] Step 2: Lithium carbonate is added to the mixture. The addition amount of lithium carbonate is calculated according to the mass of iron element added during the preparation of small particle size FePO 4 / GO, large particle size FePO 4 / GO and FePO 4 / SWCNT. Lithium carbonate is added and mixed according to a molar ratio of iron to lithium of 1:1, and then deionized water is added to form a suspension. The mass ratio of water in the suspension is 60%. After spray drying, a solid material is obtained and pre-calcined at 700°C for 8 h in a nitrogen atmosphere to form a precursor;

[0077] Step 3: The precursor is added to deionized water at a mass concentration of 1 g / L and ultrasonically dispersed for 2 h. Dopamine hydrochloride is added according to a mass ratio of dopamine hydrochloride to the precursor of 2:100, and ammonia water is added to adjust the pH to 8.5. The temperature is controlled at 40°C by water bath heating, and the stirring reaction is carried out for 6 h. After filtration, it is dried in vacuum and calcined at 700°C for 2 h in a nitrogen atmosphere to obtain the cathode material.

[0078] Comparative Example 1

[0079] Compared with Example 1, FePO 4 / SWCNT is not added during the preparation of the mixture in this comparative example. The specific steps are as follows:

[0080] Preparation of small particle size FePO 4 / GO:

[0081] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and ultrasonically disperse it for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. The ferrous sulfate solution and the graphene oxide dispersion are mixed according to a mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, phosphoric acid solution (concentration of 85 wt%) is added according to a molar ratio of iron to phosphorus of 1:1.05. The temperature is controlled at 40°C by water bath heating. According to the molar ratio of H 2 O 2 and divalent iron ions of 0.5:1, 30 wt% H 2 O 2 solution is added dropwise, and the oxidation reaction is stirred for 2 h. 20 wt% ammonia water is added dropwise to adjust the pH to 1.7, and the dropping time is 10 min. After filtration, washing and drying, small particle size FePO 4 / GO, with a particle size of 100 - 150 nm.

[0082] Preparation of large - particle - size FePO 4 / GO:

[0083] Weigh ferrous sulfate heptahydrate (FeSO 4 ·7H 2 O) and dissolve it in deionized water to form a ferrous sulfate solution with a concentration of 1 mol / L. Weigh graphene oxide and add it to deionized water, and perform ultrasonic dispersion treatment for 2 h to obtain a graphene oxide dispersion. The concentration of the graphene oxide dispersion is 5 g / L. Mix the ferrous sulfate solution and the graphene oxide dispersion according to the mass ratio of ferrous sulfate to graphene oxide of 100:3. After mixing, add phosphoric acid solution (concentration: 85 wt%) according to the molar ratio of iron to phosphorus of 1:1.05. Control the temperature at 40 °C by water - bath heating. Add 30 wt% H 2 O 2 dropwise according to the molar ratio of H 2 O 2 to divalent iron ions of 0.5:1, stir and oxidize for 2 h, add 20 wt% ammonia water dropwise to adjust the pH to 1.7, with the dropping time of 50 min. Filter, wash, and dry to obtain large - particle - size FePO 4 / GO, with a particle size of 3 - 4 μm.

[0084] Preparation of the positive electrode material:

[0085] Step 1: Mix small - particle - size FePO 4 / GO and large - particle - size FePO 4 / GO according to the mass ratio of 3:2 to form a mixed material;

[0086] Step 2: Add lithium carbonate to the mixed material. The addition amount of lithium carbonate is calculated according to the mass of iron element added during the preparation of small - particle - size FePO 4 / GO and large - particle - size FePO 4 / GO. Mix with lithium carbonate according to the molar ratio of iron to lithium of 1:1, then add deionized water to form a suspension. The mass fraction of water in the suspension is 60%. After spray - drying, obtain a solid material, and pre - calcine in a nitrogen atmosphere at 700 °C for 8 h to form a precursor;

[0087] Step 3: Add the precursor to deionized water according to a mass concentration of 1 g / L, perform ultrasonic dispersion for 2 h, add dopamine hydrochloride according to the mass ratio of dopamine hydrochloride to the precursor of 5:100, and add ammonia water to adjust the pH to 8.5. Control the temperature at 40 °C by water - bath heating, stir and react for 6 h, filter and then vacuum - dry, and calcine in a nitrogen atmosphere at 700 °C for 4 h to obtain the positive electrode material.

[0088] Comparative Example 2

[0089] Compared with Example 1, in this comparative example, no hydrochloric acid dopamine is added for secondary carbon coating during the preparation of the cathode material, and other conditions and steps are the same as those in Example 1. The specific steps for preparing the cathode material are as follows:

[0090] Step 1: Mix small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT in a mass ratio of 3:2:1 to form a mixture;

[0091] Step 2: Add lithium carbonate to the mixture. The addition amount of lithium carbonate is calculated according to the mass of iron elements added during the preparation of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT. Lithium carbonate is added and mixed according to a molar ratio of iron to lithium of 1:1, and then deionized water is added to form a suspension. The mass fraction of water in the suspension is 60%. After spray drying, a solid material is obtained, and the cathode material is obtained by calcining at 750 °C for 10 h under a nitrogen atmosphere.

[0092] Comparative Example 3

[0093] Compared with Example 1, in this comparative example, the mixture, lithium source, and polydopamine are directly mixed and then calcined to prepare the cathode material. The specific preparation steps are as follows:

[0094] Step 1: Mix small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT in a mass ratio of 3:2:1 to form a mixture;

[0095] Step 2: Add lithium carbonate to the mixture. The addition amount of lithium carbonate is calculated according to the mass of iron elements added during the preparation of small-particle-size FePO 4 / GO, large-particle-size FePO 4 / GO, and FePO 4 / SWCNT. Lithium carbonate is added and mixed according to a molar ratio of iron to lithium of 1:1, and then polydopamine is added. The mass ratio of polydopamine to the total mass of the mixture and lithium carbonate is 5:100. Then, deionized water is added to form a suspension. The mass fraction of water in the suspension is 60%. After spray drying, a solid material is obtained, and the cathode material is formed by calcining at 750 °C for 10 h under a nitrogen atmosphere.

[0096] The positive electrode materials prepared in Examples 1 - 7 and Comparative Examples 1 - 3 were subjected to compaction tests and application tests: Lithium iron phosphate positive electrode materials, PVDF, and carbon black with a mass ratio of 90:6:4 were weighed respectively, and then an appropriate amount of NMP solvent was added for dispersion. They were evenly coated on aluminum foil, dried in an oven at 100 °C for 8 h, rolled and cut into circular electrode sheets, weighed and thickness measured, and the compaction density of the electrode sheets was calculated; the electrode sheets were assembled into CR2430 type coin lithium-ion batteries for electrochemical performance testing. The test results are shown in Table 1.

[0097]

[0098] As can be seen from Table 1, in Examples 1 - 3 of the present invention, by adjusting the mixing ratio of iron phosphate materials with different particle sizes and shapes, the particle distribution of the lithium iron phosphate positive electrode material was optimized, the pores between particles were reduced, and the compaction density of the material was effectively increased. Its discharge specific capacity was relatively high, and the cycle capacity retention rate at 1C rate could reach 90%. When the amount of graphene coating increased in Examples 4 and 5, it would hinder ion transport, resulting in a decrease in discharge specific capacity and a slight reduction in cycle performance. For the secondary coating of polydopamine, in Examples 6 and 7, on the basis of Example 3, the dosage of dopamine hydrochloride was reduced during the preparation process to reduce the coating thickness. The electrochemical performance of the positive electrode material in Example 6 was significantly improved. Due to insufficient secondary coating thickness of the positive electrode material in Example 7, the cycle performance decreased. Without adding FePO 4 / SWCNT, the positive electrode material prepared in Comparative Example 1 had a lower compaction density index and inferior electrochemical performance compared to Example 1 because the distribution structure between particles was looser; in Comparative Example 2, without secondary coating, the cycle performance was poor and the capacity retention rate was lower than 85%; in Comparative Example 3, the positive electrode material prepared by direct mixing and calcination, at this time polydopamine could not be evenly coated on the surface of the precursor, and the surface of the formed secondary coating layer was uneven, which would cause problems such as cracks or peeling of the coating layer during cycling, and the cycle performance decreased significantly.

[0099] It should be noted that in this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method, article or device.

[0100] Although embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions and variations can be made to these embodiments without departing from the principles and spirit of the present invention, and the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a graphene-coated lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: Step 1, mixing small-particle FePO4 / GO, large-particle FePO4 / GO and FePO4 / SWCNT in a mass ratio of (3-5): (1-2): (0.5-1) to form a mixture; Step 2: adding lithium source and water to the mixture to form a suspension, spray drying to obtain a solid material, and pre-calcining at 700-750° C. for 6-8 hours under a nitrogen atmosphere to form a precursor; Step 3: The precursor is dispersed in water at a mass concentration of 1-3 g / L, dopamine hydrochloride and ammonia water are added, reacted at 30-40° C. for 6-8 hours, filtered and vacuum dried, and calcined at 700-750° C. for 2-4 hours under a nitrogen atmosphere to obtain a positive electrode material; The particle size of the small-particle FePO4 / GO is 100-150 nm, and the particle size of the large-particle FePO4 / GO is 3-4 μm; The small-particle FePO4 / GO and large-particle FePO4 / GO are both ferric phosphate loaded by precipitation on graphene oxide, and FePO4 / SWCNT is ferric phosphate loaded by precipitation on single-walled carbon nanotubes.

2. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The preparation steps of the small-particle FePO4 / GO are as follows: After the ferrous sulfate solution and the graphene oxide dispersion are mixed, a phosphoric acid solution is added at a molar ratio of iron to phosphorus of 1:1.05-1.08, and at 40-50° C., a 20-30wt% H2O2 solution is added dropwise at a molar ratio of H2O2 to divalent iron ions of 0.5-0.6:1, and the oxidation reaction is stirred for 1-2h, and 20-30wt% ammonia water is added dropwise to adjust the pH to 1.5-2.0, and the dropping time is 10-20min, and the small-sized FePO4 / GO is obtained by filtering, washing, and drying; The preparation steps of the large-particle FePO4 / GO are as follows: After the ferrous sulfate solution and the graphene oxide dispersion are mixed, a phosphoric acid solution is added at a molar ratio of iron to phosphorus of 1:1.05-1.08, and at 40-50°C, a 20-30wt% H2O2 solution is added dropwise at a molar ratio of H2O2 to divalent iron ions of 0.5-0.6:1, and the oxidation reaction is stirred for 1-2h, and 20-30wt% ammonia water is added dropwise to adjust the pH to 1.5-2.0, and the dropping time is 50-60min. The large-particle FePO4 / GO is obtained by filtering, washing, and drying.

3. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 2, characterized in that: The concentration of the ferrous sulfate solution is 1-2 mol / L, the concentration of the graphene oxide dispersion is 4-5 g / L, and the mass ratio of the ferrous sulfate solution to the graphene oxide dispersion is 100:3-4.

4. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The preparation steps of the FePO4 / SWCNT are as follows: After the ferrous sulfate solution and the single-walled carbon nanotube dispersion are mixed, a phosphoric acid solution is added at a molar ratio of iron to phosphorus of 1:1.05-1.08, and a 20-30wt% H2O2 solution is added dropwise at a molar ratio of H2O2 to divalent iron ions of 0.5-0.6:1 at 40-50°C. The oxidation reaction is stirred for 1-2h, and 20-30wt% ammonia water is added dropwise to adjust the pH to 1.5-2.

0. The dropping time is 10-20min, and the FePO4 / SWCNT is filtered, washed, and dried to obtain the FePO4 / SWCNT.

5. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 4, characterized in that: The single-walled carbon nanotube has a length of 1-2 μm and a diameter of 1-2 nm.

6. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 4, characterized in that: The concentration of the ferrous sulfate solution is 1-2 mol / L, the concentration of the single-walled carbon nanotube dispersion is 1-2 g / L, and the mass ratio of the ferrous sulfate solution to the single-walled carbon nanotube dispersion is 100:3-4.

7. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mixed material and the lithium source in the suspension are mixed at a molar ratio of iron to lithium of 1:1-1.02, and the mass proportion of water in the suspension is 50-60%.

8. The method for preparing a graphene-coated lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of dopamine hydrochloride to the precursor is 2-5:100; The ammonia water is used to adjust the pH of the reaction system to 8.2-8.

5.

9. A graphene-coated lithium iron phosphate positive electrode material, characterized in that: The method is described in any one of claims 1 to 8.

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