A high compaction density, low specific surface area lithium iron phosphate material and its preparation method

Abstract

The present disclosure provides a lithium iron phosphate positive electrode material, which is characterized by comprising a substrate and a coating layer; the chemical formula of the lithium iron phosphate positive electrode material is Li x Fe y Me z PO4 / C; wherein 0.97≤x≤1.03, 0.95≤y≤1.01, 0<z≤0.05, Me is selected from at least one of Mg, Ti, B, V, Zr or Nb; the coating layer is composed of at least one element of C, Li, Mg, B, Ti, P or O; the carbon content of the positive electrode material is 1.05% to 1.45%; the BET range of the substrate is: 10.6≤BET≤13.0m 2 / g; the compaction density of the lithium iron phosphate positive electrode material is 2.58~2.72g / cm 3 .

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion battery positive electrode materials, and in particular to a lithium iron phosphate material with high compaction density and low specific surface area and a preparation method thereof. Background Art

[0002] Supported by the national dual-carbon energy strategy and policies for restructuring the energy industry, my country's new energy vehicle industry and green energy storage sector are booming. However, due to factors such as limited range growth for new energy vehicles and the high cost of lithium-ion batteries, a significant number of Chinese consumers remain cautious about new energy vehicles and green sustainable energy (primarily solar and wind power). Improving the energy density of new energy vehicle batteries while reducing the unit energy cost of lithium-ion batteries are key areas for further development of the new energy sector.

[0003] The theoretical specific capacity of lithium iron phosphate cathode materials is 170 mAh / g. Currently, the voltage platform and capacity of most lithium iron phosphate cathode materials on the market are close to the theoretical values. Despite this, the energy density of the battery systems they form remains relatively low, limiting their application in power batteries and other applications. This low energy density is primarily due to their generally low compaction density. As consumers demand increasingly higher range, the demand for lithium iron phosphate with a high compaction density will increase. In other words, the key to improving the energy density of lithium iron phosphate cathode materials lies in increasing their compaction density.

[0004] Therefore, adopting a simple, efficient and low-cost method to achieve the preparation of high compaction density and low specific surface area lithium iron phosphate positive electrode materials is a very promising solution with application prospects, which urgently needs further research and exploration. Summary of the Invention

[0005] The present disclosure aims to address at least one of the technical problems existing in the above-mentioned prior art. To this end, the present disclosure provides a high compaction density, low specific surface area lithium iron phosphate material and a preparation method thereof. The preparation method can effectively increase the compaction density of the lithium iron phosphate positive electrode material and reduce its specific surface area, thereby increasing the material's energy density and improving the material's processing performance.

[0006] According to a first aspect of the present disclosure, a lithium iron phosphate positive electrode material is provided, comprising a substrate and a coating layer;

[0007] The chemical formula of the lithium iron phosphate positive electrode material is Li x Fe y Me z PO4 / C; wherein 0.97≤x≤1.03, 0.95≤y≤1.00, 0≤z≤0.05, and Me is selected from at least one of Mg, Ti, B, V, Zr, and Nb;

[0008] The coating layer is composed of at least one element selected from the group consisting of C, Li, Mg, B, Ti, V, P, and O;

[0009] The carbon content of the lithium iron phosphate positive electrode material is 1.05% to 1.45%;

[0010] The BET range of the lithium iron phosphate positive electrode material is: 10.6≤BET≤13.0m 2 / g;

[0011] The compaction density of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.58-2.72 g / cm 3 .

[0012] In this disclosure, if the mass fraction of the C element in the positive electrode material is too low, the particles will grow larger, affecting the material's capacity. If the mass fraction of the C element in the positive electrode material is too high, not only will the material's compaction density be reduced, but the proportion of active material will also be reduced, further affecting the capacity. If the BET of the positive electrode material is too large, there is a risk of gelation during battery pulping, affecting processing. If the BET is too small, the carbon content of the positive electrode material is low or the particles are large, resulting in poor capacity.

[0013] In some embodiments, the compaction density of the lithium iron phosphate positive electrode material is 2.62 to 2.72 g / cm 3 If the compaction density of the positive electrode material is too small, the energy density is low; if the compaction density is too large, the particles grow too large, affecting the capacity and the energy density is relatively low. When the compaction density is within the range described in this disclosure, the energy density of the positive electrode material can reach a relatively ideal value.

[0014] In some embodiments, the tap density of the lithium iron phosphate positive electrode material is 0.97 to 1.35 g / cm 3 The tap density of the cathode material affects its compaction density, which in turn affects the energy density.

[0015] In some embodiments, the volume particle size of the lithium iron phosphate cathode material is: 1.1 ≤ Dv50 ≤ 1.4 μm, Dv99 ≤ 8 μm. A smaller volume particle size of the cathode material results in a lower compaction density; a larger volume particle size affects the capacity of the cathode material. Within the above volume particle size range, the cathode material exhibits a superior energy density.

[0016] In some embodiments, the lithium iron phosphate cathode material has a 0.1C charge capacity of ≥162.6 mAh / g and a 0.1C discharge capacity of ≥155.6 mAh / g within a voltage range of 2.0-3.75 V. If the charge capacity of the cathode material is too low, the overall energy density of the material is low; if the charge capacity of the cathode material is too high, the particles are too fine, resulting in poor overall energy density. A charge and discharge capacity within this range is beneficial for the battery's first-cycle discharge performance and helps compensate for the loss of active lithium in the first cycle, resulting in better energy density.

[0017] In some embodiments, the coating layer has a thickness of 3 to 13 nm. The uniform coating layer improves the conductivity of the positive electrode material.

[0018] According to a second aspect of the present disclosure, a method for preparing the lithium iron phosphate positive electrode material as described in the first aspect of the present disclosure is disclosed, comprising the following steps:

[0019] S1: mixing raw materials containing an iron source, a phosphorus source, a lithium source, a composite additive containing a doping element Me, and a carbon source, performing wet grinding, and then spray drying to obtain a lithium iron phosphate spray material;

[0020] S2: sintering the lithium iron phosphate spray material in an inert atmosphere to obtain a lithium iron phosphate positive electrode material.

[0021] The disclosed process for sintering lithium iron phosphate raw materials mixed with composite additives is simple, efficient, low-cost, and easily industrializable. This process is applicable not only to lithium iron phosphate systems but also to other material systems such as lithium iron manganese phosphate, demonstrating its broad applicability. It has significant practical significance for reducing the BET of materials, improving slurry stability, enhancing material processing performance, and increasing compaction density, thereby increasing the material's energy density.

[0022] In some embodiments, the composite additive comprises a hyperdispersant PAMA and a material containing a metal element Me.

[0023] In some embodiments, the wet grinding process includes: first, mixing all raw materials uniformly with deionized water for 20-30 minutes; then, coarse grinding for 30-60 minutes to remove large particles, with large particles accounting for 10-30%; then, fine grinding the remaining slurry for 120-180 minutes; and finally, mixing the large and small particles for 30 minutes to obtain the ground slurry. This gradation of large and small particles can improve the compaction density of the material to a certain extent.

[0024] In some embodiments, in step S1, the molar ratio of the iron source, phosphorus source, and lithium source is (0.95-1):1:(1-1.03). If the iron source ratio is too low, the capacity of the positive electrode material is poor; if the iron source ratio is too high, the material's compaction density is low. If the lithium source ratio is too low, the capacity of the positive electrode material is affected; if the lithium source ratio is too high, the positive electrode material's particles are finer and the compaction density is low.

[0025] In some embodiments, in step S1, the carbon source is at least two of glucose, polyethylene glycol, polyvinyl alcohol, starch, or Super P. Using a combination of carbon sources can achieve a three-dimensional carbon coating effect, enhance the conductivity of the material, and thus improve the electrical properties of the material.

[0026] In some embodiments, the amount of the carbon source added is 10-20 wt % of the total mass of the iron source and the phosphorus source. The thickness of the lithium iron phosphate positive electrode material coating layer is controlled by the amount of the carbon source and the composite additive added.

[0027] In some embodiments, in step S1, the amount of the composite additive added is 0.1 to 10 wt % of the total mass of the iron source and the phosphorus source. If the addition ratio of the composite additive is too low, the capacity and compaction density of the positive electrode material are less effectively improved; if the addition ratio is too high, the total energy density of the positive electrode material is lower.

[0028] In some embodiments, in step S1, the iron source is at least one of ferric phosphate, ferrous oxalate, or ferric oxide.

[0029] In some embodiments, in step S1, the phosphorus source is at least one of ferric phosphate, ammonium dihydrogen phosphate, or ammonium monohydrogen phosphate.

[0030] In some embodiments, in step S1, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium phosphate, or lithium dihydrogen phosphate.

[0031] In some embodiments, in step S1, the solid content of the raw material is 30% to 50%. If the solid content of the raw material is low, it will affect the production efficiency; if the solid content is high, the viscosity of the slurry is high, which will affect the grinding process.

[0032] In some embodiments, in step S1, the inlet air temperature of the spray drying is about 200-220°C, and the outlet air temperature is about 100-110°C.

[0033] In some embodiments, in step S2, the inert atmosphere is a nitrogen atmosphere.

[0034] In some embodiments, in step S2, the sintering temperature is 780-830° C. If the sintering temperature is too low, the particles of the positive electrode material are small and the compaction density is low; if the sintering temperature is too high, the particles of the positive electrode material are large, which will affect the capacity.

[0035] In some embodiments, in step S2, the sintering holding time is 8 to 15 hours.

[0036] In some embodiments, in step S2, the lithium iron phosphate positive electrode material is further crushed and demagnetized after sintering, and the particle size Dv50 of the lithium iron phosphate positive electrode material after crushing is 1.1-1.4 μm.

[0037] According to a third aspect of the present disclosure, a composite additive is proposed, which comprises a hyperdispersant PAMA and a material containing a metal element Me, wherein the hyperdispersant PAMA is a polycarboxylic acid dispersant containing a polyether side chain; and the metal element Me is selected from at least one of Mg, Ti, B, V, Zr or Nb.

[0038] Without composite additives, the raw materials for synthesizing lithium iron phosphate are unstable during mixing, and the particles quickly sink. At this point, the particles are primarily suspended by electrostatic repulsion and the buoyancy of water, and under the influence of gravity, the particles tend to aggregate. The hyperdispersant PAMA itself contains anchoring groups, solvated chains, and pendant groups (OCH2CH2). The anchoring groups, or carboxyl groups, adsorb onto the surface of the lithium iron phosphate raw material particles, forming a negatively charged adsorption layer with electrostatic repulsion, while the solvated chains extend into the water, repelling each other and generating steric hindrance. The hydrophilic solvated chains are highly compatible with the aqueous medium, resulting in low viscosity and good fluidity for the stably dispersed lithium iron phosphate raw material particles. After the composite additive is added, the anchoring groups of the hyperdispersant PAMA are adsorbed on the surface of the lithium iron phosphate raw material particles, and the solvated chains dissolve in water, forming a solvated protective layer outside the particles. The electrostatic repulsion and steric hindrance prevent the particles from approaching each other, maintaining a stable dispersed state, thereby increasing the dispersion of the lithium iron phosphate raw materials and enhancing their stability. At the same time, the doped metal elements are evenly coated on the surface of the raw material particles with the adsorption effect of the hyperdispersant PAMA, thereby achieving the purpose of uniform doping. At the same time, the hyperdispersant PAMA and the combined carbon source are evenly coated on the surface of the particles. After sintering and cracking, a uniform thin carbon film will be formed on the surface of the particles, achieving the effect of uniform coating and improving conductivity. The presence of a small amount of doped metal elements in the coating layer is beneficial to improving the electrochemical properties of the material.

[0039] In addition, after the lithium iron phosphate positive electrode material is sintered with the addition of composite additives, the morphology of the lithium iron phosphate material becomes a regular spherical shape, which achieves the effect of reducing the specific surface area of ​​the lithium iron phosphate material; it can also increase the compaction density of the lithium iron phosphate positive electrode material, thereby significantly improving the energy density of the material.

[0040] In some embodiments, the mass proportion of the material containing the metal element Me is 4% to 11%; and / or the solid content of the composite additive is 20% to 30%. When applied to the preparation of lithium iron phosphate positive electrode materials, the amount of composite additive added is calculated based on solid mass. If the proportion of the material containing the metal element Me is too low, the capacity of the positive electrode material will not be greatly improved; if the proportion of the material containing the metal element Me is too high, the particles of the positive electrode material will be too small and the compaction density will be low. A low solid content of the composite additive will affect the effect of improving the compaction density of the positive electrode material; if the solid content is high, the viscosity of the slurry will be high and the dispersion effect of the doping elements will be poor.

[0041] In some embodiments, the composite additive is composed of a hyperdispersant PAMA and a Ti-containing material and a V-containing material, wherein the mass proportion of the Ti-containing material is 4% to 9%, and the mass proportion of the V-containing material is 0.5% to 2%. The Ti-containing material and the V-containing material in the composite additive have the effect of increasing the capacity of the lithium iron phosphate positive electrode material. If the mass proportion of the Ti-containing material is too low, the capacity improvement of the positive electrode material is limited; if the mass proportion of the Ti-containing material is too high, it will affect the improvement of the overall energy density of the positive electrode material. If the mass proportion of the V-containing material is too low, the capacity improvement of the positive electrode material is not high; if the mass proportion of the V-containing material is too high, it will have a significant adverse effect on the compaction density.

[0042] In some embodiments, the Ti-containing material is at least one of titanium dioxide or n-butyl titanate; and / or the V-containing material is at least one of vanadium pentoxide or vanadyl oxalate. Using these Ti- and C-containing raw materials does not introduce new impurities, thus avoiding any additional adverse effects on the performance of the positive electrode material.

[0043] According to a fourth aspect of the present disclosure, a method for preparing the composite additive according to the third aspect of the present disclosure is provided, comprising the following steps:

[0044] Acid anhydride and poly (fatty alcohol) monomethyl ether are mixed and heated once. After the reaction, carboxylic acid and materials containing metal element Me are added and heated again. The initiator is added while maintaining the temperature. The temperature is continued to rise and the reaction is continued. The composite additive is obtained after the pH is adjusted to neutral or alkaline.

[0045] Adding materials containing doping elements during the preparation of the hyperdispersant effectively improves the dispersibility of the doping elements, which is conducive to obtaining positive electrode materials with better doping and coating effects.

[0046] In some embodiments, the mass proportion of the material containing the metal element Me is 5% to 10%.

[0047] In some embodiments, the molar ratio of the added amounts of the acid anhydride, poly(fatty alcohol monomethyl ether), and carboxylic acid is 1:(0.48-0.52):(0.9-1.1).

[0048] In some embodiments, the molar ratio of the acid anhydride to the initiator is 1:(1.0-1.2).

[0049] In some embodiments, the primary heating temperature is 85-100° C. and the time is 2-3 hours.

[0050] In some embodiments, the secondary heating temperature is 75-80° C. and the time is 2-3 hours.

[0051] In some embodiments, the temperature-raising reaction is performed at a temperature of 80-85° C., and the reaction time is 2-2.5 h.

[0052] In some embodiments, the step of adjusting the pH to neutral or alkaline is adjusting the pH to 7-8 using sodium hydroxide.

[0053] According to a fifth aspect of the present disclosure, a positive electrode plate is provided, comprising the lithium iron phosphate positive electrode material as described in the first aspect of the present disclosure.

[0054] According to a sixth aspect of the present disclosure, a lithium-ion battery is provided, comprising the positive electrode sheet as described in the fifth aspect.

[0055] According to one embodiment of the present disclosure, there are at least the following beneficial effects:

[0056] (1) The lithium iron phosphate positive electrode material disclosed in the present invention has the characteristics of high compaction density and low specific surface area, which makes the material have a high energy density. At the same time, the low specific surface area also makes the material have good processing performance.

[0057] (2) The lithium iron phosphate positive electrode material disclosed in the present invention has a higher compaction density and better electrochemical performance due to the better doping amount and uniform doping and coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] The present disclosure is further described below with reference to the accompanying drawings and embodiments, wherein:

[0059] Figure 1 This is a SEM image of the lithium iron phosphate positive electrode material prepared in Example 1 of the present disclosure;

[0060] Figure 2This is a SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 1 of the present disclosure;

[0061] Figure 3 This is an SEM image of the lithium iron phosphate positive electrode material prepared in Comparative Example 2 of the present disclosure. DETAILED DESCRIPTION

[0062] The following will clearly and completely describe the concept and technical effects of the present disclosure in conjunction with embodiments to fully understand the purpose, features and effects of the present disclosure.

[0063] In the data of the following examples and comparative examples, the carbon content is measured using a high-frequency infrared carbon-sulfur analyzer in accordance with GB / T 20123-2006 / ISO15350:2000 & JJG395-2016; the specific surface area is measured using the BET method for gas adsorption of solid substances in accordance with GB / T 19587-2017; the compacted density is measured at a pressure of 3 T in accordance with GB / T 24533-2009; the tapped density is measured in accordance with GB / T 5162-2006, Determination of tapped density of metal powders; and the volume particle size is measured in accordance with GB / T 19077-2016 / ISO13320:2009.

[0064] Example 1

[0065] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.996, z=0.004, Me is selected from Ti and V, wherein Ti is 0.003, V is 0.001, and the mass fraction of C element in the positive electrode material is 1.251%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-8nm. The BET of the positive electrode material is 10.9m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.706g / cm 3 , the tap density TD is 1.06g / cm 3 The particle size D50 is 1.32μm, the particle size D99 is 6.62μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 162.8mAh / g, and the 0.1C discharge specific capacity is 156.6mAh / g.

[0066] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material, comprising the following steps:

[0067] (1) Preparation of lithium iron phosphate spray material: 90 kg of anhydrous iron phosphate, 22.64 kg of lithium carbonate, 4.5 kg of glucose and 5.4 kg of polyethylene glycol (molecular weight 1500) were taken, 180 kg of deionized water were added and mixed for 30 minutes, 25 kg of composite additives (solid content 20%) were taken and poured into the above mixture and continued to mix for 30 minutes, and then put into a sand mill, first coarsely ground to a particle size of 1.3 μm, the coarse grinding speed was set to 500 rpm, large particles were taken out, and their weight accounted for 20%, and then finely ground, the fine grinding speed was also set to 500 rpm, and the small particles were ground to 0.4 μm, that is, small particles were obtained. Finally, the large and small particles were mixed for 30 minutes, and the medium particle size of the mixed particles was 0.49 μm. Then, spray drying was carried out, and the inlet air temperature was 220 ° C and the outlet air temperature was 110 ° C to obtain the spray material.

[0068] (2) Preparation of sintered lithium iron phosphate cathode material: The spray material obtained in step (1) is placed in a graphite crucible and placed in a roller kiln for sintering. The high temperature section temperature is set to 810°C and the high temperature section holding time is set to 10 hours. Nitrogen is passed through the sintering process for protection. After cooling, the material is crushed and demagnetized to obtain a lithium iron phosphate cathode material with high compaction density and low specific surface area. The SEM image of the lithium iron phosphate cathode material is as follows: Figure 1 shown.

[0069] A composite additive is used in the preparation method of the above-mentioned lithium iron phosphate positive electrode material, and its composition by mass percentage is: 90% hyperdispersant PAMA, 9% titanium dioxide, and 1% vanadyl oxalate, and the solid content of the composite additive is 20%.

[0070] The preparation method of the composite additive as described above comprises the following steps:

[0071] To a three-necked flask equipped with a stirring, reflux and condensing apparatus, 0.98 kg of maleic anhydride and 2.5 kg of polyethylene glycol monomethyl ether (molecular weight 500) were added, and the mixture was reacted at 85° C. for 2 h. After the reaction, the mixture was cooled, and then 1.44 L of an aqueous acrylic acid solution (50% by mass), 0.45 kg of titanium dioxide and 0.05 kg of vanadyl oxalate were added. The temperature was raised to 75° C., and 4.56 L of an aqueous ammonium persulfate solution (50% by mass) was added dropwise to initiate polymerization. After the initiator was added, the temperature was raised to 85° C., the reaction was continued for 2 h, and the pH value was adjusted to 7 with an appropriate amount of sodium hydroxide to obtain a light yellow, slightly viscous liquid with a solid content of 20%.

[0072] Example 2

[0073] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Mez PO4 / C, wherein x=1.00, y=0.9936, z=0.0064, and Me is selected from Ti and V, wherein Ti is 0.0048 and V is 0.0016. The mass fraction of the C element in the positive electrode material is 1.352%; the coating layer is mainly composed of C, and the thickness of the coating layer is 6 to 12 nm; the BET of the positive electrode material is 11.3m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.693g / cm 3 , the tap density TD is 1.01g / cm 3 The particle size D50 is 1.29μm, the particle size D99 is 6.71μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.1mAh / g, and the 0.1C discharge specific capacity is 156.8mAh / g.

[0074] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that in step (1), 8 kg of the composite additive is dissolved in 20 kg of water. Other conditions and parameters are exactly the same as in Example 1.

[0075] The above composite additive and its preparation method are the same as those in Example 1.

[0076] The SEM image of the lithium iron phosphate cathode material prepared in Example 2 is as follows: Figure 2 shown.

[0077] Example 3

[0078] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.998, z=0.002, and Me is selected from Ti and V, wherein Ti is 0.0015 and V is 0.0005. The mass fraction of the C element in the positive electrode material is 1.162%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-7nm; the BET of the positive electrode material is 10.8m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.688g / cm 3 , the tap density TD is 1.04g / cm 3 The particle size D50 is 1.24μm, the particle size D99 is 6.73μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 162.9mAh / g, and the 0.1C discharge specific capacity is 156.7mAh / g.

[0079] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that in step (1), 2.5 kg of the composite additive is dissolved in 20 kg of water, and the other conditions and parameters are exactly the same as those in Example 1.

[0080] The above composite additive and its preparation method are the same as those in Example 1.

[0081] Example 4

[0082] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.999, z=0.001, and Me is selected from Ti and V, wherein Ti is 0.00075 and V is 0.00025. The mass fraction of the C element in the positive electrode material is 1.164%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-7nm; the BET of the positive electrode material is 11.1m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.647g / cm 3 , the tap density TD is 0.99g / cm 3 The particle size D50 is 1.21μm, the particle size D99 is 5.72μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.1mAh / g, and the 0.1C discharge specific capacity is 156.7mAh / g.

[0083] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that in step (1), 1.25 kg of the composite additive is dissolved in 20 kg of water, and the other conditions and parameters are exactly the same as those in Example 1.

[0084] The above composite additive and its preparation method are the same as those in Example 1.

[0085] Example 5

[0086] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.996, z=0.004, and Me is selected from Ti and V, wherein Ti is 0.003 and V is 0.001. The mass fraction of the C element in the positive electrode material is 1.263%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-9 nm; the BET of the positive electrode material is 11.2m2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.695g / cm 3 , the tap density TD is 1.01g / cm 3 The particle size D50 is 1.25μm, the particle size D99 is 5.79μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.8mAh / g, and the 0.1C discharge specific capacity is 157.2mAh / g.

[0087] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that the temperature of the high-temperature section in step (2) is set to 808°C, and the other conditions and parameters are exactly the same as those in Example 1.

[0088] The above composite additive and its preparation method are the same as those in Example 1.

[0089] Example 6

[0090] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.996, z=0.004, and Me is selected from Ti and V, wherein Ti is 0.003 and V is 0.001. The mass fraction of the C element in the positive electrode material is 1.242%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-8nm; the BET of the positive electrode material is 10.8m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.713g / cm 3 , the tap density TD is 1.12g / cm 3 The particle size D50 is 1.36μm, the particle size D99 is 6.82μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.2mAh / g, and the 0.1C discharge specific capacity is 156.4mAh / g.

[0091] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that the temperature of the high-temperature section in step (2) is set to 812° C., and other conditions and parameters are exactly the same as those in Example 1.

[0092] The above composite additive and its preparation method are the same as those in Example 1.

[0093] Example 7

[0094] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.9936, z=0.0064, and Me is selected from Ti and V, wherein Ti is 0.0048 and V is 0.0016. The mass fraction of the C element in the positive electrode material is 1.336%; the coating layer is mainly composed of C, and the thickness of the coating layer is 6 to 11 nm; the BET of the positive electrode material is 11.1 m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.698g / cm 3 , the tap density TD is 1.12g / cm 3 The particle size D50 is 1.31μm, the particle size D99 is 6.72μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.4mAh / g, and the 0.1C discharge specific capacity is 156.4mAh / g.

[0095] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 2 is that the temperature of the high-temperature section in step (2) is set to 812° C., and other conditions and parameters are exactly the same as those in Example 2.

[0096] The above composite additive and its preparation method are the same as those in Example 2.

[0097] Example 8

[0098] This embodiment provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.9936, z=0.0064, and Me is selected from Ti and V, wherein Ti is 0.0048 and V is 0.0016. The mass fraction of the C element in the positive electrode material is 1.232%; the coating layer is mainly composed of C, and the thickness of the coating layer is 7 to 13 nm; the BET of the positive electrode material is 11.4 m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.658g / cm 3 , the tap density TD is 0.98g / cm 3 The particle size D50 is 1.22μm, the particle size D99 is 6.51μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 163.2mAh / g, and the 0.1C discharge specific capacity is 156.8mAh / g.

[0099] This embodiment also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 2 is that the temperature of the high-temperature section in step (2) is set to 808°C, and the other conditions and parameters are exactly the same as those in Example 2.

[0100] The above composite additive and its preparation method are the same as those in Example 2.

[0101] Comparative Example 1

[0102] This comparative example provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=1, z=0.00. The mass fraction of C element in the positive electrode material is 1.211%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-9nm; the BET of the positive electrode material is 12.9m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.536g / cm 3 , the tap density TD is 0.87g / cm 3 The particle size D50 is 1.32μm, the particle size D99 is 7.32μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 161.1mAh / g, and the 0.1C discharge specific capacity is 154.6mAh / g.

[0103] This comparative example also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material, which differs from Example 1 only in that no composite additive is added in step (1), and other conditions and parameters are exactly the same as those in Example 1.

[0104] The SEM image of the lithium iron phosphate cathode material prepared in Comparative Example 1 is as follows: Figure 3 shown.

[0105] Comparative Example 2

[0106] This comparative example provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.996, z=0.004, and Me is selected from Ti and V, wherein Ti is 0.003 and V is 0.001. The mass fraction of the C element in the positive electrode material is 1.153%; the coating layer is mainly composed of C, and the thickness of the coating layer is 4-8nm; the BET of the positive electrode material is 12.6m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.549g / cm 3 , the tap density TD is 0.86g / cm 3 The particle size D50 is 1.22μm, the particle size D99 is 7.12μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 161.8mAh / g, and the 0.1C discharge specific capacity is 155.2mAh / g.

[0107] This comparative example also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that no composite additive is added in step (1), 450g of titanium dioxide and 50g of vanadyl oxalate are additionally added during the mixing process, and the other conditions and parameters are exactly the same as those in Example 1.

[0108] Comparative Example 3

[0109] This comparative example provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=1, z=0.00. The mass fraction of C element in the positive electrode material is 1.236%; the coating layer is mainly composed of C, and the thickness of the coating layer is 5-10nm; the BET of the positive electrode material is 11.3m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.596g / cm 3 , the tap density TD is 0.95g / cm 3 The particle size D50 is 1.36μm, the particle size D99 is 7.43μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 161.6mAh / g, and the 0.1C discharge specific capacity is 154.4mAh / g.

[0110] This comparative example also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that no composite additive is added in step (1), 4.5 kg of hyperdispersant PAMA is additionally added during the mixing process, and other conditions and parameters are exactly the same as in Example 1.

[0111] The preparation method of the hyperdispersant PAMA as described above comprises the following steps:

[0112] 0.98 kg of maleic anhydride and 2.5 kg of polyethylene glycol monomethyl ether (molecular weight 500) were added to a three-necked flask equipped with a stirring, reflux and condensing device, and the mixture was reacted at 85°C for 2 hours. After the reaction, the mixture was cooled, and then 1.44 L of an aqueous acrylic acid solution (mass concentration 50%) was added. The temperature was raised to 75°C, and 4.56 L of an aqueous ammonium persulfate solution (mass concentration 50%) was added dropwise to initiate polymerization. After the initiator was added, the temperature was raised to 85°C, the reaction was continued for 2 hours, and the pH value was adjusted to 7 with an appropriate amount of sodium hydroxide to obtain a hyperdispersant PAMA.

[0113] Comparative Example 4

[0114] This comparative example provides a lithium iron phosphate positive electrode material, including a substrate and a coating layer. The chemical formula of the lithium iron phosphate positive electrode material is: Li x Fe y Me z PO4 / C, wherein x=1.00, y=0.996, z=0.004, and Me is selected from Ti and V, wherein Ti is 0.003 and V is 0.001. The mass fraction of the C element in the positive electrode material is 1.240%; the coating layer is mainly composed of C, and the thickness of the coating layer is 6 to 12 nm; the BET of the positive electrode material is 11.8 m 2 / g, and the compaction density PD of the lithium iron phosphate positive electrode material at a pressure of 3T is 2.696g / cm 3 , the tap density TD is 1.07g / cm 3 The particle size D50 is 1.26μm, the particle size D99 is 6.81μm, and in the voltage range of 2.0-3.75V, the 0.1C charge specific capacity is 162.0mAh / g, and the 0.1C discharge specific capacity is 155.1mAh / g.

[0115] This comparative example also provides a method for preparing the above-mentioned lithium iron phosphate positive electrode material. The only difference from Example 1 is that no composite additive is added in step (1), and 4.5 kg of hyperdispersant PAMA, 450 g of titanium dioxide and 50 g of vanadyl oxalate are added during the mixing process. Other conditions and parameters are exactly the same as in Example 1.

[0116] The preparation method of the hyperdispersant PAMA as described above is the same as that of Comparative Example 3.

[0117] Test example

[0118] The lithium iron phosphate positive electrode materials prepared in Examples 1-8 and Comparative Examples 1-4 were subjected to compaction density, tap density, BET tests, and a buckling test in a voltage range of 2.0-3.75V.

[0119] Performance test steps: The lithium iron phosphate positive electrode material, conductive carbon black (SP) conductive agent, polyvinylidene fluoride (PVDF) and an appropriate amount of N-methylpyrrolidone (NMP) described in the present disclosure are mixed evenly by a high-speed mixer, wherein the mass ratio of the lithium iron phosphate positive electrode material, SP conductive agent, and PVDF is 90:5:5. Then, an automatic coating machine is used to coat the aluminum foil, and the coated aluminum foil is dried in an oven and cut into small discs of the required size and weighed; a pure lithium sheet is used as the negative electrode sheet, and the positive electrode shell, negative electrode shell, positive electrode sheet, lithium sheet, separator, and electrolyte are assembled into a button battery as required; the button battery is hung on the battery testing system and tested after standing: the test results are summarized in Table 1.

[0120] Table 1

[0121]

[0122] As can be seen from Table 1, the difference between the examples and the comparative examples lies mainly in whether the composite additives are added. However, the compaction density and specific surface area of ​​the obtained lithium iron phosphate cathode materials are quite different. Among them, the difference between Example 1 and Example 2 lies in the different amounts of composite additives. The amount added in Example 2 is higher, and the capacity of the obtained lithium iron phosphate cathode material is higher. Although the compaction density is slightly lower than that of Example 1, it is still at a relatively high level. Figure 1 、 Figure 2 Comparing the morphologies of the two, it can be seen that the surface of the lithium iron phosphate positive electrode material prepared in Example 2 is smoother, indicating that increasing the amount of composite additives is beneficial to improving the effect of carbon coating.

[0123] The difference between Example 1 and Comparative Example 1 is that in Example 1, a composite additive is added during the preparation of the lithium iron phosphate positive electrode material, and it can be seen that both the compaction density and the capacity are greatly improved. Figure 1 、 Figure 3 Comparing the morphologies of the two, it can be seen that the lithium iron phosphate positive electrode material prepared in Example 1 has better sphericity, a smoother surface, and a better carbon coating effect.

[0124] The difference between Example 1 and Comparative Example 2 is that the composite additive added in Example 1 contains the hyperdispersant PAMA, and the compaction density of the obtained positive electrode material is greatly improved; although the same amount of dopants (titanium dioxide and vanadium oxalate) is also added to Comparative Example 2, the capacity is still lower than that of Example 1.

[0125] The difference between Example 1 and Comparative Example 3 is that in Comparative Example 3, only an equal amount of hyperdispersant PAMA is added without adding a dopant, and the capacity of the prepared lithium iron phosphate positive electrode material is lower.

[0126] The difference between Example 1 and Comparative Example 4 is that in Example 1, the doping element is first dispersed in the hyperdispersant PAMA to form a composite additive before mixing. In Comparative Example 4, equal amounts of the hyperdispersant PAMA and the dopant are added separately during the mixing process, resulting in a lower capacity lithium iron phosphate cathode material than in Example 1. This is because the composite additive has a better dispersion effect, which in turn makes the doping more uniform and the doping effect better.

[0127] After adding the composite additives, the capacity of the lithium iron phosphate positive electrode material increases. The reason is that the anchoring group of the hyperdispersant PAMA is adsorbed on the surface of the lithium iron phosphate raw material particles, and the solvated chains are dissolved in water to form a solvated protective layer outside the particles. The electrostatic repulsion and steric hindrance prevent the particles from approaching each other, maintaining a stable dispersed state, thereby increasing the dispersion of the lithium iron phosphate raw materials and enhancing their stability. At the same time, the doped Ti and V elements are evenly wrapped on the surface of the raw material particles with the adsorption of the hyperdispersant PAMA, thereby achieving the purpose of uniform doping.

[0128] In addition, after adding the composite additives, the BET of the lithium iron phosphate cathode material is reduced, and the compaction density is increased, and the energy density is also greatly improved accordingly. The reason is that the addition of the composite additives helps to achieve uniform dispersion of the doping elements and uniform coating of the carbon source after sintering. In conjunction with the sintering process, the stress of the lithium iron phosphate material is greatly reduced, the density is increased, and the compaction is effectively improved. For lithium iron phosphate cathode materials synthesized with different ingredient formulas, the compaction density is greatly improved when treated using the process disclosed in this invention, indicating that the process has the characteristics of wide adaptability.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: including a base material and a coating layer; The chemical formula of the lithium iron phosphate positive electrode material is Li x Fe y Me z PO4 / C; wherein, 0.97≤x≤1.03, 0.95≤y≤1.00, 0<z≤0.05, and Me is selected from at least one of Mg, Ti, B, V, Zr or Nb; The coating layer is composed of at least one element selected from the group consisting of C, Li, Mg, B, Ti, V, P, and O; The carbon content of the lithium iron phosphate positive electrode material is 1.05% to 1.45%; The BET range of the lithium iron phosphate positive electrode material is: 10.6≤BET≤13.0 m 2 / g; The compaction density of the lithium iron phosphate positive electrode material at 3T pressure is 2.58-2.72 g / cm 3 ; The tap density of the lithium iron phosphate positive electrode material is 0.97-1.35 g / cm 3 ; The preparation method of the lithium iron phosphate positive electrode material comprises the following steps: S1: mixing raw materials containing an iron source, a phosphorus source, a lithium source, a composite additive containing a doping element Me, and a carbon source, performing wet grinding, and then spray drying to obtain a lithium iron phosphate spray material; S2: sintering the lithium iron phosphate spray material in an inert atmosphere to obtain a lithium iron phosphate positive electrode material; The composite additive comprises a hyperdispersant PAMA and a material containing a doping element Me, wherein the hyperdispersant PAMA is a polycarboxylic acid dispersant containing a polyether side chain; the mass proportion of the material containing the doping element Me in the composite additive is 4% to 11%; and / or the solid content of the composite additive is 20% to 30%; The preparation method of the composite additive comprises the following steps: Acid anhydride and poly (fatty alcohol) monomethyl ether are mixed and heated once. After the reaction, carboxylic acid and materials containing doping element Me are added and heated again. The initiator is added while maintaining the temperature. The temperature is continued to rise and the reaction is continued. The composite additive is obtained after the pH is adjusted to neutral or alkaline.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The compaction density of the lithium iron phosphate positive electrode material is 2.62-2.72 g / cm 3 .

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The volume particle size of the lithium iron phosphate positive electrode material is: 1.1≤Dv50≤1.4 μm, and Dv99≤8 μm.

4. The lithium iron phosphate cathode material according to claim 1, characterized in that The lithium iron phosphate positive electrode material has a 0.1C charge specific capacity of 162.6-164.3 mAh / g and a 0.1C discharge specific capacity of 155.6-160.3 mAh / g in a voltage range of 2.0-3.75 V.

5. The lithium iron phosphate positive electrode material according to claim 1, characterized in that The thickness of the coating layer is 3-13 nm.

6. A method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: mixing raw materials containing an iron source, a phosphorus source, a lithium source, a composite additive containing a doping element Me, and a carbon source, performing wet grinding, and then spray drying to obtain a lithium iron phosphate spray material; S2: sintering the lithium iron phosphate spray material in an inert atmosphere to obtain a lithium iron phosphate positive electrode material; The preparation method of the composite additive comprises the following steps: Acid anhydride and poly (fatty alcohol) monomethyl ether are mixed and heated once. After the reaction, carboxylic acid and materials containing doping element Me are added and heated again. The initiator is added while maintaining the temperature. The temperature is continued to rise and the reaction is continued. The composite additive is obtained after the pH is adjusted to neutral or alkaline.

7. The preparation method according to claim 6, characterized in that In step S1, the molar ratio of the iron source, the phosphorus source, and the lithium source is (0.95-1):1:(1-1.03).

8. The preparation method according to claim 6, characterized in that In step S1, the carbon source is at least two of glucose, polyethylene glycol, polyvinyl alcohol, starch or Super P.

9. The preparation method according to claim 6, characterized in that In step S1, the added amount of the composite additive is 0.1-10 wt % of the total mass of the iron source and the phosphorus source.

10. The preparation method according to claim 6, characterized in that In step S1, the solid content of the raw material is 30% to 50%.

11. The preparation method according to claim 6, characterized in that In step S2, the sintering temperature is 780-830°C.

12. Use of a composite additive in preparing a lithium iron phosphate positive electrode material, characterized in that: The composite additive comprises a hyperdispersant PAMA and a material containing a doping element Me, wherein the hyperdispersant PAMA is a polycarboxylic acid dispersant containing a polyether side chain; the doping element Me is selected from at least one of Mg, Ti, B, V, Zr, and Nb; the mass proportion of the material containing the doping element Me is 4% to 11%; and / or the solid content of the composite additive is 20% to 30%; The preparation method of the lithium iron phosphate positive electrode material comprises: mixing raw materials containing an iron source, a phosphorus source, a lithium source, a composite additive containing a doping element Me, and a carbon source, performing wet grinding, and then spray drying to obtain a lithium iron phosphate spray material; The preparation method of the composite additive comprises the following steps: Acid anhydride and poly (fatty alcohol) monomethyl ether are mixed and heated once. After the reaction, carboxylic acid and materials containing doping element Me are added and heated again. The initiator is added while maintaining the temperature. The temperature is continued to rise and the reaction is continued. The composite additive is obtained after the pH is adjusted to neutral or alkaline.

13. The use according to claim 12, characterized in that The composite additive consists of a hyperdispersant PAMA, a Ti-containing material, and a V-containing material. The mass proportion of the Ti-containing material is 4-9%, and the mass proportion of the V-containing material is 0.5-2%.

14. The use according to claim 13, characterized in that The Ti-containing material is at least one of titanium dioxide and n-butyl titanate; and / or the V-containing material is at least one of vanadium pentoxide and vanadyl oxalate.

15. A positive electrode plate, characterized in that: The invention comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 5.

16. A lithium ion battery, characterized in that: Contains the positive electrode sheet as claimed in claim 15.

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