Positive electrode composite material, preparation method therefor and use thereof

Abstract

A positive electrode composite material, a preparation method therefor and a use thereof. The positive electrode composite material comprises a nano lithium manganese iron phosphate material, a first coating layer coated on the surface of the nano lithium manganese iron phosphate material, and a second coating layer coated on the surface of the first coating layer; the first coating layer comprises a nano ternary active material; and the second coating layer is a carbon coating layer. The conductivity and the structural stability of the lithium manganese iron phosphate material are improved.

Description

A positive electrode composite material and its preparation method and application

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on November 23, 2023, with application number 2023115813189. The entire contents of the above application are incorporated by reference into this application. Technical Field

[0002] The present application relates to the technical field of lithium batteries, and in particular to a positive electrode composite material and a preparation method and application thereof. Background Art

[0003] In recent years, with the rapid development of new energy technologies, the research and development of power batteries has been in full swing. The focus of power battery research and development has gradually shifted to the development of high-energy-density battery materials. Although ternary materials have excellent high specific capacity, there are certain concerns about the safety of ternary lithium batteries. While materials such as lithium iron phosphate (LIFP) have stable structures and good safety, their low specific energy leads to a short driving range for passenger cars using LFP batteries, which cannot meet people's travel needs. Therefore, there is an urgent need to find a new lithium battery material that can balance high energy density and safety.

[0004] Both lithium iron manganese phosphate and lithium iron phosphate have olivine structures and excellent thermodynamic stability, which makes lithium iron manganese phosphate batteries have similar safety as lithium iron phosphate batteries. In addition, lithium iron manganese phosphate batteries have energy density similar to that of ternary lithium batteries. Therefore, lithium iron manganese phosphate can be used as a lithium battery material for the development of both high energy density and high safety performance.

[0005] However, it is worth noting that compared with the transition energy gap of 0.3eV of lithium iron phosphate, the transition energy gap of electrons in lithium manganese iron phosphate is as high as 2eV, which is basically an insulator and has the disadvantages of low electronic conductivity and low ion mobility; moreover, manganese elements will be dissolved in lithium manganese iron phosphate during the battery cycle, resulting in a decrease in the structural stability of lithium manganese iron phosphate, which directly limits the development and application of lithium manganese iron phosphate batteries. Technical issues

[0006] In order to improve the conductivity and structural stability of lithium manganese iron phosphate materials, the present application provides a positive electrode composite material and a preparation method and application thereof.

[0007] Technical Solution

[0008] In a first aspect, the present application provides a positive electrode composite material, comprising a nano-manganese iron phosphate lithium material, a first coating layer coated on the surface of the nano-manganese iron phosphate lithium material, and a second coating layer coated on the surface of the first coating layer;

[0009] The first coating layer includes nano ternary active material, and the second coating layer is a carbon coating layer.

[0010] In a second aspect, the present application provides a method for preparing a positive electrode composite material.

[0011] A method for preparing a positive electrode composite material comprises the following steps:

[0012] Step 1: adding an iron source, a first manganese source, and a first lithium source to a first aqueous solvent under stirring, and then adding a phosphorus source, stirring and dissolving, and then performing a hydrothermal reaction to obtain a nano-manganese iron lithium phosphate material;

[0013] Step 2: Mixing a nickel source, a second manganese source, a cobalt source, a second lithium source, and an ammonia salt with a second aqueous solvent, stirring and dissolving the mixture, and then mixing the mixture with the nano-manganese iron lithium phosphate material, performing a hydrothermal reaction and then annealing to form a first coating layer on the surface of the nano-manganese iron lithium phosphate material;

[0014] Step 3: The nano-lithium manganese iron phosphate material with the first coating layer formed on its surface obtained in step 2 is mixed with an organic carbon source solution, taken out, dried, and then annealed to obtain the positive electrode composite material.

[0015] In a third aspect, the present application provides a positive electrode sheet, comprising a positive current collector and a positive electrode material layer disposed on a surface of the positive current collector, wherein the positive electrode material layer comprises a conductive agent, a binder and the positive electrode composite material as described above.

[0016] In a fourth aspect, the present application provides a lithium-ion battery, comprising the positive electrode sheet as described above. Beneficial effects

[0017] 1. By nano-sizing lithium manganese iron phosphate materials, more electrochemical reaction areas and shorter ion transmission paths can be provided, thereby increasing the material's specific capacity, reducing the degree of structural damage during the charge and discharge process to weaken the attenuation of the specific capacity, and overcoming the poor conductivity of pure lithium manganese iron phosphate materials, thereby improving the material's rate performance.

[0018] 2. It can improve the degree of bonding between the lithium manganese iron phosphate material and the nano ternary active material during the coating process, obtain a first coating layer with excellent coating effect, and make the ternary active material constituting the first coating layer have a more complete layered structure. The more complete the layered structure, the better the stability of the first coating layer, which is manifested as an improvement in the cycle performance and safety performance of the positive electrode composite material.

[0019] 3. The presence of the first coating layer can increase the content of intercalable and deintercalable lithium ions in the positive electrode material, further improving the specific capacity of the positive electrode composite material. In addition, the nano-ternary active material in the first coating layer complements the performance of the lithium manganese iron phosphate material, producing a synergistic effect, which further improves the conductivity and structural stability of the lithium manganese iron phosphate material.

[0020] 4. Introduce carbon material into the nano-ternary active material of the first coating layer and form a second coating layer, utilizing the good conductivity and stability of the carbon material to reduce the internal resistance, improve the charge transfer capability, and further improve the rate characteristics and cycle stability of the lithium manganese iron phosphate material.

[0021] 5. The presence of the second coating layer and the first coating layer can also effectively prevent the lithium manganese iron phosphate material from directly contacting the electrolyte during the battery cycle, avoid manganese dissolution from the lithium manganese iron phosphate material, and make the positive electrode composite material have higher structural stability, thereby improving its cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG1 is a DCIR data diagram of lithium batteries corresponding to Example 1 and Comparative Examples 1-3.

[0023] FIG2 is a discharge test diagram of the lithium batteries corresponding to Example 1 and Comparative Examples 1-3 at 25° C. and 5C rate. Modes for Carrying Out the Invention

[0024] Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, and so forth used in the specification and claims are to be understood as modified by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth herein are approximations that can vary depending upon the desired properties sought to be obtained.

[0025] As used herein, "and / or" means one or all of the mentioned elements.

[0026] As used herein, “including” and “comprising” encompasses the case where only the stated elements are present and also the case where there are other elements other than the stated elements.

[0027] All percentages in this application are by weight unless otherwise stated.

[0028] Unless otherwise indicated, as used in this specification, "a," "an," "an," and "the" are intended to include "at least one" or "one or more." For example, "a component" refers to one or more components, and thus more than one component is contemplated and may be employed or used in the practice of the described embodiment.

[0029] In one embodiment, the thickness of the first coating layer is 1 / 20-1 / 10 of the particle size D50 of the nano-lithium manganese iron phosphate material.

[0030] In one embodiment, the thickness of the second coating layer is 1 / 800-1 / 500 of the particle size D50 of the nano-lithium manganese iron phosphate material.

[0031] By adjusting the thickness of the first coating layer and the second coating layer, while achieving effective coating of the nano-lithium manganese iron phosphate material, phase separation at the two-phase interface between the carbon coating layer, the first coating layer and the lithium manganese iron phosphate material will not occur. This thickness can make the above-mentioned positive electrode composite material have good structural stability, high ionic conductivity and electronic conductivity, thereby improving the conductivity of the positive electrode composite material.

[0032] In one embodiment, the particle size D50 of the nano-lithium manganese iron phosphate material is 600-1200 nm.

[0033] By controlling the particle size of the nano-manganese iron phosphate lithium material, the thickness of the first coating layer and the second coating layer can be controlled within a reasonable range, so that the particle size of the finally prepared positive electrode composite material will not be too small or too large, and it has higher structural stability.

[0034] In one embodiment, the nano ternary active material is a high-nickel nano ternary active material, and the molar fraction of nickel in the high-nickel nano ternary active material is 0.8-0.9.

[0035] By controlling the molar fraction of nickel in high-nickel nano-ternary active materials, the specific capacity of the positive electrode sheet using the positive electrode composite material is increased, and the energy density of the manganese iron phosphate battery is increased.

[0036] In one embodiment, the carbon coating layer comprises a carbon material, wherein the carbon material is selected from an organic carbon source, and the organic carbon source is selected from at least one of glucose, sucrose, soluble starch, citric acid, β-cyclodextrin, and polyvinyl alcohol.

[0037] The specific surface area of ​​the positive electrode composite material is 20-25m 2 / g.

[0038] By controlling the specific surface area of ​​the positive electrode composite material, the positive electrode composite material can exhibit good bonding performance with binders, conductive agents and other materials in the positive electrode sheet, which is beneficial for lithium batteries to obtain excellent conductivity and cycle performance.

[0039] In one embodiment, the mass ratio of the iron source, the first manganese source, the first lithium source and the phosphorus source in step 1 is 25-29:16-18:6-10:10-15; the temperature of the hydrothermal synthesis in step 1 is 85-95° C., and the hydrothermal reaction time is 220-260 min.

[0040] In one embodiment, in step one, the iron source includes at least one of FeSO4, FeCl2, FeC204.2H2O, Fe(OH)3, and Fe(NO3)3; the first manganese source includes at least one of MnSO4, MnCO3, and MnCl2; the first lithium source includes at least one of Li2CO3, LiOH, and LiCl; the phosphorus source includes at least one of H3PO4, H3PO3, and LiH2PO4, wherein the phosphoric acid refers to a phosphoric acid aqueous solution with a mass concentration of 60%-85%; and the first aqueous solvent is water or an aqueous solution containing ions and an organic solvent.

[0041] In one embodiment, the mass ratio of the nickel source, the second manganese source, the cobalt source, the second lithium source and the ammonium salt in step 2 is 450-650:20-60:40-100:150-260:20-50; the temperature of the hydrothermal synthesis in step 2 is 110-130° C., and the hydrothermal reaction time is 340-380 min; the annealing temperature in step 2 is 500-600° C., and the annealing time is 10-14 h.

[0042] In one embodiment, in step 2, the nickel source includes at least one of NiSO4, Ni(OH)2, and Ni(NO3)2; the second manganese source includes at least one of MnSO4, MnCO3, and MnCl2; the cobalt source includes at least one of Co(NO3)2, CoSO4, and CoCl2; the second lithium source includes at least one of Li2CO3, LiOH, and LiCl; the ammonia salt includes NH4F; and the second aqueous solvent is water or an aqueous solution containing ions and an organic solvent.

[0043] In one embodiment, the concentration of the organic carbon source solution in step 3 is 0.01-0.02 mol / L; the annealing temperature in step 3 is 700-800° C., and the annealing time is 7-9 h.

[0044] By preparing highly dispersed nano-lithium manganese iron phosphate materials with uniform precursor morphology through hydrothermal reaction, the nucleation time window can be reduced, the synthesized nano-lithium manganese iron phosphate materials can be evenly suspended and dispersed in the solution, and the nucleation points can be increased, thereby preparing a positive electrode composite material with excellent coating performance of the first coating layer and the second coating layer.

[0045] Example 1

[0046] 1. Preparation of cathode composite materials

[0047] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 240 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 900nm, centrifuge the nano-manganese iron lithium phosphate material and dry and grind it for later use;

[0048] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution; mix the nano-manganese iron phosphate material obtained in step 1 with the second solution, ultrasonically disperse for 30min to obtain a suspension; hydrothermally react the suspension at 120°C for 360min to obtain nano-ternary active material-coated lithium manganese iron phosphate, the molar fraction of nickel in the ternary active material is 0.85 and the thickness of the first coating layer composed of the ternary active material is 60nm; the lithium manganese iron phosphate coated with the ternary active material is centrifuged, dried, ground, and annealed at 550°C for 12h to obtain a nano-composite material;

[0049] Step 3: Soak the nano-manganese iron lithium phosphate material with the first coating layer in 0.01 mol / L glucose solution for 30 min and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in 95 vol% Ar+5 vol% H2 atmosphere at 750°C for 8 h to obtain a specific surface area of ​​22.7 m 2 / g of positive electrode composite material, and the thickness of the second coating layer in the positive electrode composite material is 1.5nm.

[0050] 2. Preparation of positive electrode

[0051] The positive electrode composite material and SP:PVDF:CNT were mixed and stirred at a mass ratio of 96.5:0.9:2:0.6, and NMP was used as the solvent to produce a positive electrode slurry. The above positive electrode slurry was evenly coated on a 12μm thick carbon-coated aluminum foil and dried at 90°C to produce a dry positive electrode sheet.

[0052] 3. Preparation of negative electrode sheet

[0053] Artificial graphite, SP, CMC and SBR were mixed and stirred in a mass ratio of 95.5:2:0.5:2, and deionized water was used as a solvent to obtain a negative electrode slurry. The negative electrode slurry was then coated on a copper foil with a thickness of 8 μm, and the electrode was vacuum dried at 100°C.

[0054] 4. Preparation of lithium-ion batteries

[0055] 4.1 Preparation of electrolyte

[0056] The electrolyte was prepared in an argon-filled glove box, where the water content in the glove box was less than 10 ppm and the oxygen content was less than 1 ppm. The electrolyte was prepared using the following steps: 1.2 mol / L LiPF6 and LFSI (LiPF6:LFSI=1.1:0.1) were prepared in a volume ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC) = 30:40:30, and then 0.5 wt% of the additives VC, 1.5 wt% of FEC, 0.5 wt% of DTD, and 0.5 wt% of LiPO2F2 were added, mixed evenly and set aside.

[0057] 4.2 Lithium-ion Battery Assembly

[0058] The vacuum-dried positive and negative electrodes and separators were punched out and then assembled into CR2032 button batteries in an argon-filled glove box.

[0059] The DCIR data of the lithium battery in this embodiment is shown in FIG1 ; the discharge performance of the lithium battery in this embodiment at 25° C. and 5C rate is shown in FIG2 .

[0060] Example 2

[0061] 1. Preparation of cathode composite materials

[0062] Step 1: Add 25g FeSO4.7H2O, 18g MnSO4.H2O, and 6g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 10.5ml of 60% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 95°C for 220 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 600nm, centrifuge the nano-manganese iron lithium phosphate material and dry and grind it for later use;

[0063] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution; mix the nano-manganese iron phosphate material obtained in step 1 with the second solution, ultrasonically disperse for 30min to obtain a suspension; hydrothermally react the suspension at 110°C for 380min to obtain nano-ternary active material-coated lithium manganese iron phosphate, the molar fraction of nickel in the ternary active material is 0.8 and the thickness of the first coating layer composed of the ternary active material is 30nm; centrifuge, dry, grind and anneal the lithium manganese iron phosphate coated with the ternary active material, and then anneal at 500°C for 14h to obtain a nano-composite material;

[0064] Step 3: Soak the nano-manganese iron lithium phosphate material with the first coating layer in a 0.015 mol / L citric acid solution for 30 min and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in a 95 vol% Ar+5 vol% H2 atmosphere at 700°C for 9 h to obtain a specific surface area of ​​20.5 m 2 / g of positive electrode composite material, and the thickness of the second coating layer in the positive electrode composite material is 1.2nm.

[0065] 2. Preparation of positive electrode

[0066] The preparation of the positive electrode is the same as that in Example 1.

[0067] 3. Preparation of negative electrode sheet

[0068] The preparation of the negative electrode sheet is the same as that in Example 1.

[0069] 4. Preparation of lithium-ion batteries

[0070] The assembly of the lithium battery is the same as that in Example 1.

[0071] Example 3

[0072] 1. Preparation of cathode composite materials

[0073] Step 1: Add 29g FeSO4.7H2O, 16g MnSO4.H2O, and 10g Li2CO3 into a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 8.5ml of 60% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, and hydrothermally react the first mixed solution at 85°C for 260 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 1200nm. The nano-manganese iron lithium phosphate material is centrifuged and dried and ground for later use;

[0074] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution; mix the nano-manganese iron phosphate material obtained in step 1 with the second solution, ultrasonically disperse for 30min to obtain a suspension; hydrothermally react the suspension at 130°C for 340min to obtain nano-ternary active material-coated lithium manganese iron phosphate, the molar fraction of nickel in the ternary active material is 0.9 and the thickness of the first coating layer composed of the ternary active material is 120nm; the lithium manganese iron phosphate coated with the ternary active material is centrifuged, dried, ground, and annealed at 600°C for 10h to obtain a nano-composite material;

[0075] Step 3: Soak the nano-manganese iron lithium phosphate material with the first coating layer in a 0.02 mol / L sucrose solution for 30 min and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in a 95 vol% Ar + 5 vol% H2 atmosphere at 800 ° C for 7 h to obtain a specific surface area of ​​24.8 m 2 / g of positive electrode composite material, and the thickness of the second coating layer in the positive electrode composite material is 1.5nm.

[0076] 2. Preparation of positive electrode

[0077] The preparation of the positive electrode is the same as that in Example 1.

[0078] 3. Preparation of negative electrode sheet

[0079] The preparation of the negative electrode sheet is the same as that in Example 1.

[0080] 4. Preparation of lithium-ion batteries

[0081] The assembly of the lithium battery is the same as that in Example 1.

[0082] Example 4

[0083] The difference between this embodiment and embodiment 1 is that the thickness of the first coating layer in the positive electrode composite material is 1 / 25 of the particle size of the nano-lithium manganese iron phosphate material; the thickness of the second coating layer is 1 / 300 of the particle size of the nano-lithium manganese iron phosphate material; specifically, the steps of preparing the positive electrode composite material using the nano-lithium manganese iron phosphate material in steps 2 and 3 are as follows:

[0084] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 240 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 900nm, centrifuge the nano-manganese iron lithium phosphate material and dry and grind it for later use;

[0085] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution; mix the nano-manganese iron phosphate material obtained in step 1 with the second solution, ultrasonically disperse for 30min to obtain a suspension; hydrothermally react the suspension at 120°C for 160min to obtain nano-ternary active material-coated lithium manganese iron phosphate, the molar fraction of nickel in the ternary active material is 0.85 and the thickness of the first coating layer composed of the ternary active material is 36nm; the lithium manganese iron phosphate coated with the ternary active material is centrifuged, dried, ground, and annealed at 550°C for 12h to obtain a nano-composite material;

[0086] Step 3: Soak the nano-manganese iron phosphate material with the first coating layer in a 0.01 mol / L glucose solution for 120 min and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in a 95 vol% Ar+5 vol% H2 atmosphere at 750°C for 1 h to obtain a specific surface area of ​​34 m 2 / g of positive electrode composite material, and the thickness of the second coating layer in the positive electrode composite material is 3nm.

[0087] The rest of the parts are consistent with those in Example 1.

[0088] Example 5

[0089] The difference between this embodiment and embodiment 1 is that the thickness of the first coating layer in the positive electrode composite material is 1 / 5 of the particle size of the nano-lithium manganese iron phosphate material; the thickness of the second coating layer is 1 / 1000 of the particle size of the nano-lithium manganese iron phosphate material; specifically, the steps of preparing the positive electrode composite material using the nano-lithium manganese iron phosphate material in steps 2 and 3 are as follows:

[0090] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 240 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 900nm, centrifuge the nano-manganese iron lithium phosphate material and dry and grind it for later use;

[0091] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution; mix the nano-manganese iron phosphate material obtained in step 1 with the second solution, ultrasonically disperse for 30min to obtain a suspension; hydrothermally react the suspension at 120°C for 600min to obtain nano-ternary active material-coated lithium manganese iron phosphate, the molar fraction of nickel in the ternary active material is 0.85 and the thickness of the first coating layer composed of the ternary active material is 180nm; the lithium manganese iron phosphate coated with the ternary active material is centrifuged, dried, ground, and annealed at 550°C for 12h to obtain a nano-composite material;

[0092] Step 3: Soak the nano-manganese iron phosphate material with the first coating layer in a 0.01 mol / L glucose solution for 20 min and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in a 95 vol% Ar+5 vol% H2 atmosphere at 750°C for 1 h to obtain a specific surface area of ​​18 m 2 / g positive electrode composite material, and the thickness of the second coating layer in the positive electrode composite material is 0.9nm.

[0093] Example 6

[0094] The difference between this embodiment and embodiment 1 is that the particle size D50 of the nano-lithium manganese iron phosphate material prepared in step 1 is 400 nm. Specifically, the preparation of the nano-lithium manganese iron phosphate material in step 1 is as follows:

[0095] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 into a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 90 minutes to obtain nano-manganese iron lithium phosphate material, centrifuge the nano-manganese iron lithium phosphate material, and dry and grind it for later use.

[0096] The final specific surface area was 13m 2 / g positive electrode composite material; the rest are consistent with Example 1.

[0097] Example 7

[0098] The difference between this embodiment and embodiment 1 is that the particle size D50 of the nano-lithium manganese iron phosphate material prepared in step 1 is 1500 nm. Specifically, the preparation of the nano-lithium manganese iron phosphate material in step 1 is as follows:

[0099] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 into a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 360 minutes to obtain nano-manganese iron lithium phosphate material, centrifuge the nano-manganese iron lithium phosphate material, and dry and grind it for later use.

[0100] The final surface area was 10m 2 / g positive electrode composite material; the rest are consistent with Example 1.

[0101] Comparative Example 1

[0102] The difference between this comparative example and Example 1 lies in the preparation of the positive electrode composite material. The positive electrode composite material is prepared by the following steps:

[0103] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 240 minutes to obtain a nano-manganese iron lithium phosphate material, centrifuge the nano-manganese iron lithium phosphate material, and dry and grind it for later use;

[0104] Step 2: Add 5.6g NiSO4.7H2O, 0.43g MnSO4.H2O, 0.73g Co(NO3)2.6H20, 2g Li2CO3 and 0.37g NH4F into a 200ml beaker, then add 60ml deionized water and 20ml ethanol, stir and dissolve for 10min to obtain a second mixed solution, and hydrothermally react the second mixed solution at 120°C for 360min to obtain a nano ternary active material;

[0105] Step 3: Physically mix the nano-manganese iron lithium phosphate material prepared in step 1 with the nano-ternary active material prepared in step 2 to prepare a positive electrode composite material.

[0106] The rest of the parts are consistent with those in Example 1.

[0107] The DCIR data of the lithium battery in this comparative example is shown in FIG1 ; the discharge performance of the lithium battery in this comparative example at 25° C. and 5C rate is shown in FIG2 .

[0108] Comparative Example 2

[0109] The difference between this comparative example and Example 1 lies in the preparation of the positive electrode composite material. Step 3 is omitted in the preparation of the positive electrode composite material. The rest of the steps are consistent with Example 1.

[0110] The DCIR data of the lithium battery in this comparative example is shown in FIG1 ; the discharge performance of the lithium battery in this comparative example at 25° C. and 5C rate is shown in FIG2 .

[0111] Comparative Example 3

[0112] The difference between this comparative example and Example 1 lies in the preparation of the positive electrode composite material. The positive electrode composite material is prepared by the following steps:

[0113] Step 1: Add 27g FeSO4.7H2O, 17g MnSO4.H2O, and 8g Li2CO3 to a 200ml beaker, add 100ml deionized water and 20ml ethanol, and finally add 6ml of 85% H3PO4 aqueous solution, stir and dissolve in a nitrogen atmosphere for 10 minutes to obtain a first mixed solution, hydrothermally react the first mixed solution at 90°C for 240 minutes to obtain a nano-manganese iron lithium phosphate material with a particle size D50 of 900nm, centrifuge the nano-manganese iron lithium phosphate material and dry and grind it for later use;

[0114] Step 2: Soak the nano-manganese iron lithium phosphate material obtained in step 1 in a 0.01 mol / L glucose solution for 30 minutes and ultrasonically disperse it. Take out the soaked nano-composite material, centrifuge it, dry it, and anneal it in a 95 vol% Ar+5 vol% H2 atmosphere at 750°C for 8 hours to obtain a positive electrode composite material.

[0115] The rest of the parts are consistent with those in Example 1.

[0116] The DCIR data of the lithium battery in this comparative example is shown in FIG1 ; the discharge performance of the lithium battery in this comparative example at 25° C. and 5C rate is shown in FIG2 .

[0117] Detection method

[0118] 1. DCIR test

[0119] The lithium-ion batteries of Examples 1-7 and Comparative Examples 1-3 were subjected to a DCIR test. The specific test method is as follows: the lithium battery capacity was calibrated at 1C at 25°C, and the state of charge (SOC) was adjusted to 50%. Then, all the cells were discharged at 1C for 30 seconds at 50% SOC, and the discharge DCIR was calculated. The test results are shown in Figure 1.

[0120] 2. Rate performance

[0121] The lithium-ion batteries of Examples 1-7 and Comparative Examples 1-3 were subjected to a 5C rate discharge test at 25° C., with a charge and discharge voltage range of 2.5V-4.25V. Specific test results are shown in FIG2 .

[0122] 3. Cycle performance test

[0123] The lithium-ion batteries of Examples 1-7 and Comparative Examples 1-3 were subjected to a cycle performance test. The specific test method is as follows: after the lithium batteries were left at 25°C for 1 hour, they were cycled with 3C step fast charge and 1C discharge. The charge and discharge voltage range was 2.5V-4.25V, and the capacity retention rate and DCIR change rate were calculated; wherein, DCIR growth rate = (500-cycle DCIR-initial DCIR) × 100% / initial DCIR, capacity retention rate = 500-cycle discharge capacity × 100% / initial discharge capacity; the test results are shown in Table 1.

[0124] Table 1

[0125] No. Cycle 500-week capacity retention rate / % Cycle 500-week DCIR growth rate / % Example 193.414.2 Example 292.864.3 Example 392.144.4 Example 491.885.2 Example 590.564.8 Example 689.765.5 Example 788.394.6 Comparative Example 187.2314.5 Comparative Example 290.058.0 Comparative Example 384.4028.0

[0126] Combining Examples 1-3, Comparative Examples 1-3, Figures 1-2, and Table 1, it can be seen that Example 1 has the lowest discharge DCIR value, only 0.6 mΩ, while the discharge DCIR values ​​of Comparative Examples 1-3 are increased by 63.3%, 35%, and 120%, respectively, compared to the discharge DCIR values ​​of Example 1. Furthermore, Example 1 has the highest cycle capacity retention rate and the lowest DCIR growth rate. This indicates that the positive electrode composite material formed by using nano-lithium manganese iron phosphate material as a crystal nucleus, growing the ternary active material at a nanoscale on the surface of the lithium manganese iron phosphate material, and then performing a carbon coating process can greatly reduce the material impedance, improve the stability of the positive electrode sheet, and fully enhance the power performance of the lithium battery. It has excellent conductivity and stability.

[0127] In combination with Example 1, Examples 4-5 and Table 1, it can be seen that when the thickness of the first coating layer and the second coating layer is selected to be too large or too small, it is not conducive to the stable bonding between the carbon coating layer, the first coating layer and the lithium manganese iron phosphate material, thereby having a certain negative impact on the conductivity of the positive electrode composite material.

[0128] In combination with Example 1, Examples 6-7 and Table 1, it can be seen that when the particle size of the nano-manganese iron phosphate lithium material is too high or too low, it will affect its specific surface area, which is not conducive to the formation of a first coating layer of appropriate thickness on the surface of the nano-three-active element material, and also affects the further coating of the second coating layer, thereby being not conducive to the formation of a positive electrode composite material with appropriate specific surface area; when the specific surface area of ​​the positive electrode composite material is too small, the positive electrode composite material will be applied in the positive electrode sheet as a battery with high internal resistance, low discharge platform, low capacity utilization, poor rate performance, and poor cycle performance. When the specific surface area of ​​the positive electrode composite material is too large, the positive electrode composite material is highly active, easy to agglomerate, difficult to disperse, and difficult to process the electrode sheet.

Claims

1. A positive electrode composite material, comprising a nano-lithium manganese iron phosphate material, a first coating layer coated on the surface of the nano-lithium manganese iron phosphate material, and a second coating layer coated on the surface of the first coating layer; the first coating layer comprises a nano-ternary active material, and the second coating layer is a carbon coating layer.

2. The positive electrode composite material according to claim 1, wherein The thickness of the first coating layer is 1 / 20-1 / 10 of the particle size D50 of the nano-lithium manganese iron phosphate material.

3. The positive electrode composite material according to claim 1, wherein The thickness of the second coating layer is 1 / 800-1 / 500 of the particle size D50 of the nano-lithium manganese iron phosphate material.

4. The positive electrode composite material according to any one of claims 1 to 3, wherein: The particle size D50 of the nano-lithium manganese iron phosphate material is 600-1200nm.

5. The positive electrode composite material according to claim 1, wherein The nano ternary active material is a high-nickel nano ternary active material, and the molar fraction of nickel in the high-nickel nano ternary active material is 0.8-0.

9.

6. The positive electrode composite material according to claim 1, wherein The carbon coating layer comprises a carbon material, wherein the carbon material is selected from an organic carbon source, and the organic carbon source is selected from at least one of glucose, sucrose, soluble starch, citric acid, β-cyclodextrin, and polyvinyl alcohol.

7. The positive electrode composite material according to claim 1, wherein: The specific surface area of ​​the positive electrode composite material is 20-25m 2 / g.

8. The method for preparing the positive electrode composite material according to any one of claims 1 to 7, comprising the following steps: Step 1, adding an iron source, a first manganese source, and a first lithium source into a first aqueous solvent under stirring, and then adding a phosphorus source, stirring and dissolving, and then performing a hydrothermal reaction to obtain a nano-manganese iron lithium phosphate material; Step 2, mixing the nickel source, the second manganese source, the cobalt source, the second lithium source and the ammonium salt with the second aqueous solvent, stirring and dissolving them, and then mixing them with the nano-manganese iron lithium phosphate material, performing annealing after hydrothermal reaction, and forming a first coating layer on the surface of the nano-manganese iron lithium phosphate material; Step three: Mix the nano-lithium manganese iron phosphate material with the first coating layer formed on the surface obtained in step two with the organic carbon source solution, take it out, dry it, and then anneal it to obtain the positive electrode composite material.

9. A positive electrode sheet, wherein: The positive electrode sheet comprises a positive current collector and a positive electrode material layer disposed on the surface of the positive current collector, wherein the positive electrode material layer comprises a conductive agent, a binder and the positive electrode composite material according to any one of claims 1 to 7. 10 . A lithium ion battery, comprising the positive electrode sheet according to claim 9 .

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