Lithium iron phosphate-coated C composite material and combined double-spraying preparation method and application thereof

Abstract

The invention belongs to the field of positive electrode materials, and particularly relates to a method for preparing a lithium iron phosphate C composite material by combining double spraying, which comprises the following steps: carrying out spray pyrolysis treatment on slurry of a lithium source, an iron source and a phosphorus source to obtain a transformed activated precursor; wherein the atmosphere in the spray pyrolysis stage is an oxygen-containing atmosphere, and the temperature is 600-900 DEG C; and carrying out composite pulpifying on the transformed and activated precursor and a carbon source, and then carrying out spray drying and heat treatment to prepare the lithium iron phosphate C composite material. The invention also discloses the material prepared by the preparation method and application thereof. The method can be based on a brand new process, anion residues are reduced, and the phase and distribution uniformity of elements are improved, so that the electrochemical performance, especially the low-temperature performance, of the material can be remarkably improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of cathode materials for lithium-ion batteries, and particularly relates to a preparation method of lithium iron phosphate. Technical Background

[0002] Since researchers such as Goodenough proposed in 1997 that LiMPO4 materials can be used as cathode materials for lithium-ion batteries, phosphate-based cathode materials have received extensive attention from scientific research workers. LiFePO4 materials have been successfully mass-produced commercially as cathode materials for power batteries due to their wide raw material sources, low cost, environmental friendliness, good thermal stability, and excellent safety performance.

[0003] Currently, the mainstream processes for preparing lithium iron phosphate are as follows: 1. Carbothermal reduction method using iron phosphate and lithium carbonate as the main raw materials; 2. High-temperature solid-phase method using ferrous oxalate, ammonium dihydrogen phosphate, and lithium carbonate as the main raw materials; 3. Self-heating evaporation liquid-phase synthesis method using iron nitrate and lithium dihydrogen phosphate as the main raw materials. Each method has high requirements for the purity and particle size of raw materials, complex processes, and high costs.

[0004] For example, most of the existing lithium iron phosphate preparation processes directly prepare lithium iron phosphate in one pot after mixing all the materials (such as lithium, iron, phosphorus, and carbon source). As disclosed in the Chinese patent document with the publication number CN116565180A, a preparation method of the high tap density lithium iron phosphate cathode material is disclosed. After mixing a lithium source, an iron source, a phosphorus source, a carbon source, a cation dopant, and an anion dopant and grinding them, spray drying is carried out to obtain a precursor material; after sintering the precursor material, the high tap density lithium iron phosphate cathode material is obtained.

[0005] Another example is the Chinese patent document with the publication number CN 111129636A, which discloses a regeneration method of the cathode material of a waste lithium iron phosphate battery. A carbon source is added to the mixed solution A containing a lithium salt and iron phosphate to control the carbon content of the lithium iron phosphate product to 1% - 10% to obtain a mixed solution B; the mixed solution B is subjected to spray pyrolysis under certain temperature conditions and an inert gas atmosphere to obtain a carbon-coated lithium iron phosphate material.

[0006] It can be seen that most of the existing technologies prepare lithium iron phosphate materials through one-time spray drying - heat treatment or spray pyrolysis of all raw materials. This preparation method is prone to anion residues, and the system stability in the carbon coating stage is difficult to control. The phase purity and distribution uniformity of the prepared materials need to be improved, which in turn leads to unsatisfactory electrochemical performance, especially low-temperature performance, of the prepared materials. Summary of the Invention

[0007] Aiming at the defects and deficiencies of the current process for preparing lithium iron phosphate, the first object of the present invention is to provide a method for preparing lithium iron phosphate @C composite materials by combined dual spraying, aiming to provide a lithium iron phosphate @C composite material that is applicable to chloride raw materials, can solve residual chlorine, improve the system stability in the carbon coating stage, improve the uniformity of product element distribution, and further improve the electrochemical performance of the material, especially the low-temperature performance.

[0008] The second object of the present invention is to provide the lithium iron phosphate @C composite material prepared by the above preparation method and its application in lithium secondary batteries.

[0009] The third object of the present invention is to provide a lithium secondary battery containing the lithium iron phosphate @C composite material, as well as its positive electrode and positive electrode material.

[0010] Aiming at the problems of unsatisfactory raw material adaptability, difficult control of system stability in the carbon coating stage, unsatisfactory uniformity of product element distribution, and unsatisfactory electrochemical performance, especially low-temperature performance, of the existing lithium iron phosphate @C materials during preparation, the present invention provides the following solutions:

[0011] A method for preparing lithium iron phosphate @C composite materials by combined dual spraying, in which a slurry of a lithium source, an iron source, and a phosphorus source is subjected to spray pyrolysis treatment to obtain a transformed and activated precursor; wherein, the atmosphere in the spray pyrolysis stage is an oxygen-containing atmosphere, and the temperature is 600 - 900 °C;

[0012] Then, the transformed and activated precursor and the carbon source composite slurry are spray-dried and heat-treated to obtain the lithium iron phosphate @C composite material.

[0013] In the present invention, innovatively, the raw materials without a carbon source are first subjected to spray pyrolysis treatment (the first spray), and then compounded with the carbon source for the second-stage spray drying - heat treatment (the second spray). In this way, synergy can be achieved, the precursor can be transformed and activated, and the residual of raw material anions can be reduced. In addition, it is helpful to improve the controllability and stability of the system in the carbon coating stage, improve the phase and distribution uniformity of the target product, and thus unexpectedly improve the electrochemical performance, especially the low-temperature performance, of the prepared material.

[0014] In the present invention, the process combination of first performing spray pyrolysis on the carbon-source-free raw materials and then spray drying - heat treatment with carbon addition is the key to synergistically solving the problem of anion residue (such as residual chlorine introduced by chloride raw materials), improving the system stability in the carbon coating stage, improving element distribution, and further improving the low-temperature performance of the material.

[0015] In the present invention, the lithium source and the iron source are water-soluble salts of their respective metal elements, preferably at least one of chlorides, acetates, sulfates, and organic acid salts.

[0016] In the prior art, lithium sources and iron sources are mainly sulfates and nitrates, and chlorides are less used. The important problem is that it is difficult to solve the residue problem of the materials prepared from chloride raw materials in the prior art. However, in the present invention, through the combined double-spray process, the residual chlorine problem can be well solved, the element distribution uniformity can be improved, and materials with better low-temperature performance can be obtained. That is to say, the process described in the present invention has a better raw material adaptation broad spectrum.

[0017] In the present invention, the phosphorus source can be a raw material that can provide phosphate radicals and is well-known in the industry. For example, in the present invention, the phosphorus source is a compound that can ionize PO 4 3- and is preferably at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate;

[0018] In the present invention, in the slurry, the molar ratio of Li, Fe, and P can be adjusted based on known principles. For example, it can be 1.01-1.06:1:1.01-1.04;

[0019] In the present invention, the solute in the slurry is composed of a lithium source, an iron source, and a phosphorus source;

[0020] In the present invention, the solvent in the slurry is water;

[0021] In the present invention, in the slurry, the concentration of iron element is 0.4-2.5 mol / L, and further can be 0.5-1 M.

[0022] In the present invention, the oxygen-containing atmosphere is at least one of oxygen, air, and a mixture of oxygen and a protective gas. The protective gas is at least one of inert gases such as nitrogen and argon;

[0023] In the present invention, the temperature of spray pyrolysis is 750-800 °C.

[0024] In the present invention, the atomization pressure of spray pyrolysis is 0.5-3 MPa;

[0025] In the present invention, the time of spray pyrolysis is 5-90 s, and further can be 30-60 s.

[0026] In the present invention, on the basis of spray pyrolysis of the carbon-source-free material, the pyrolysis product and the carbon source are compounded for the second-stage spray drying heat treatment, which is beneficial to improving the anion residue, improving the element distribution uniformity, and improving the low-temperature performance of the prepared material.

[0027] In the present invention, there is no special requirement for the type of the carbon source. For example, the carbon source is at least one of glucose, starch, sucrose, polyethylene glycol, polyvinyl alcohol, and cyclodextrin;

[0028] In the present invention, the weight ratio of the carbon source to the precursor is 0.1 to 0.3:1, and further can be 0.1 to 0.2:1.

[0029] In the present invention, the inlet temperature in the spray drying stage is 200 to 300 °C, and the outlet temperature is 70 to 120 °C.

[0030] In the present invention, the atmosphere in the heat treatment stage is at least one of inert gases such as nitrogen and argon, hydrogen, alkanes, and alkenes;

[0031] In the present invention, the temperature in the heat treatment stage is 650 to 800 °C, and further can be 700 to 750 °C;

[0032] In the present invention, the heat treatment time is 4 to 24 h, and further can be 5 to 10 h.

[0033] The present invention also provides a lithium iron phosphate @C composite material prepared by the method described above;

[0034] The preparation method of the present invention can endow the prepared material with special microscopic characteristics, and the material with such characteristics can exhibit better electrochemical performance.

[0035] For example, the lithium iron phosphate @C composite material prepared by the preparation method of the present invention includes primary lithium iron phosphate particles in the shape of spheres or quasi-spheres and secondary particles formed by their aggregation, as well as an amorphous carbon layer coating the surfaces of the primary particles and the secondary particles;

[0036] The particle size of the primary particles is 200 to 800 nm, preferably 400 to 500 nm;

[0037] The particle size of the secondary particles is 5 to 25 μm;

[0038] The thickness of the amorphous carbon layer is less than 10 nm, preferably 2 to 10 nm, and further preferably 3 to 6 nm;

[0039] Preferably, in lithium iron phosphate, the molar ratio of Fe / P is controlled at 0.96 to 0.99, and the molar ratio of Li / P is controlled at 1.01 to 1.06; the thickness of the amorphous carbon layer is 2 to 10 nm, and the amorphous carbon content is 1.0 to 2.5 wt.%, preferably 1.5 to 2 wt.%.

[0040] The present invention also provides an application of the lithium iron phosphate @C composite material prepared by the method described above. Using it as a positive electrode active material to prepare a lithium secondary battery.

[0041] In the present invention, based on known methods, the lithium iron phosphate @C composite material prepared by the present invention can be used as a positive electrode active material to prepare the required lithium secondary battery, its positive electrode, and positive electrode material.

[0042] The present invention also provides a positive electrode for a lithium secondary battery, comprising a current collector and a positive electrode material compounded on its surface, and the positive electrode material contains the lithium iron phosphate @C composite material prepared by the method described above;

[0043] In the positive electrode material, a conductive agent and a binder are further included;

[0044] In the positive electrode material, the content of the lithium iron phosphate @C composite material is above 60 wt.%, preferably 75 - 95 wt.%.

[0045] The present invention also provides a lithium secondary battery, comprising an electric core in which a positive electrode, a separator, and a negative electrode are compounded in sequence, and the positive electrode is the positive electrode containing the lithium iron phosphate @C composite material prepared by the preparation method of the present invention.

[0046] For the lithium secondary battery of the present invention, except for containing the lithium iron phosphate @C composite material prepared by the preparation method of the present invention, other structures and materials can be well-known.

[0047] Beneficial effects

[0048] In the present invention, innovatively, the raw materials without a carbon source are first subjected to spray pyrolysis treatment (the first spray), and then compounded with a carbon source for the second-stage spray drying - heat treatment (the second spray). In this way, synergy can be achieved, the precursor can be transformed and activated, and the residue of anions can be reduced. In addition, it helps to improve the controllability and stability of the system in the carbon coating stage, improve the phase and distribution uniformity of the target product, and thus unexpectedly improve the electrochemical performance, especially the low-temperature performance, of the prepared material. Description of the drawings

[0049] Figure 1 It is the XRD pattern of the pretreated precursor in Example 1.

[0050] Figure 2 It is the scanning electron microscope image of the finally prepared lithium iron phosphate in Example 1.

[0051] Figure 3 It is the XRD pattern of the finally prepared lithium iron phosphate in Example 1. Detailed implementation manners

[0052] The technical solutions of the present invention will be further described below through specific implementation manners. Those skilled in the art should understand that the described embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention. The protection scope of the present invention is subject to the claims.

[0053] Example 1

[0054] (a) Lithium chloride, iron chloride, and phosphoric acid were dissolved and mixed by heating and stirring in a certain proportion with deionized water as the solvent to obtain a pretreatment solution. In the pretreatment solution, the molar ratio of Li:Fe:P elements was 1.06:1:1.03, and the iron element concentration was 0.5 mol / L.

[0055] (b) The pretreatment solution obtained in step (a) was subjected to spray pyrolysis to obtain a pretreatment precursor. Among them, the temperature of spray pyrolysis was 750 °C; the carrier gas for spray pyrolysis was air; the pressure of spray pyrolysis was 0.9 Mpa; the time of spray pyrolysis was 30 s; the XRD of the product is shown in Figure 1 , showing the presence of Li 3 Fe 2 (PO 4 ) 3 and the hematite phase;

[0056] (c) The pretreatment precursor obtained in step (b) was mixed with glucose (carbon source, and the weight ratio of it to the pretreatment precursor was 0.15:1), and deionized water was supplemented. After stirring and dispersing and wet ball milling in sequence, a precursor slurry with a solid content of 35 wt.% was obtained.

[0057] (d) The precursor slurry obtained in step (c) was spray-dried (the inlet temperature was 280 °C and the outlet temperature was 100 °C) to obtain a precursor.

[0058] (e) The precursor obtained in step (d) was sintered at 700 °C for 6 h under a nitrogen atmosphere to obtain the lithium iron phosphate composite material. The SEM and XRD are shown in Figure 2 and Figure 3 .

[0059] The primary particle morphology of the prepared lithium iron phosphate composite material was spherical-like, the primary particle size was 500 nm, and the lithium iron phosphate composite material included lithium iron phosphate and a surface-coated carbon layer. The thickness of the surface-coated carbon layer was 5 nm, and the carbon content in the lithium iron phosphate composite material was 2 wt%.

[0060] Example 2

[0061] Compared with Example 1, the difference was only that the ratio of P and Li in step (a) and the solution concentration were changed, so that in the pretreatment solution, the molar ratio of Li:Fe:P elements was 1.02:1:1.01, and the iron element concentration was 1 mol / L. Other operations and parameters were the same as those in Example 1.

[0062] Example 3

[0063] Compared with Example 1, the difference was only that in step (b), the temperature of spray pyrolysis was 900 °C and the carrier gas was oxygen. Other operations and parameters were the same as those in Example 1.

[0064] Example 4

[0065] Compared with Example 1, the only difference is that in step (c), the carbon source is sucrose, and the weight ratio of sucrose to the pretreated precursor is 0.2:1. Other operations and parameters are the same as those in Example 1.

[0066] Example 5

[0067] Compared with Example 1, the only difference is that in step (e), the calcination temperature is 750 °C and sintering is carried out for 5 h. Other operations and parameters are the same as those in Example 1.

[0068] Example 6

[0069] Compared with Example 1, the only difference is that in step (a), lithium acetate is used as the lithium source and iron acetate is used as the iron source. Other operations and parameters are the same as those in Example 1.

[0070] Comparative Example 1

[0071] Compared with Example 1, the only difference is that all raw materials (Li / Fe / P and carbon raw materials) are subjected to one-step spraying treatment (spray drying - heat treatment), specifically: the spray pyrolysis treatment process in step b is omitted, but the glucose in step c is mixed in step a (the ratio of Li-P-Fe and the carbon source in the obtained slurry is the same as that in Example 1), and then the treatments in steps d and e are carried out. Other operations and parameters are the same as those in Example 1.

[0072] Comparative Example 2

[0073] Compared with Comparative Example 1, the only difference is that all raw materials (Li / Fe / P and carbon raw materials) are subjected to one-step spraying treatment (spray pyrolysis), specifically: steps c - e are omitted, but the carbon source in step c is mixed in step a (the ratio of Li-P-Fe and the carbon source in the obtained slurry is the same as that in Example 1), and then the slurry containing the lithium source, iron source, phosphorus source and carbon source is subjected to one-step spray pyrolysis treatment, and the atmosphere is Ar. Other conditions are the same as those in step b of Example 1.

[0074] Comparative Example 3

[0075] Compared with Example 1, the only difference is that steps a and b are not carried out, and in step c, lithium phosphate, iron phosphate and iron oxide red are used as the pretreated precursor (the phase composition of the pretreated precursor in this case is close to that in Example 1, and the Li, Fe, P ratio is the same as that in Example 1), and step c and subsequent treatments are carried out. Other operations and parameters are the same as those in Example 1.

[0076] Comparative Example 4

[0077] Compared with Example 1, a two-stage spray drying-thermal treatment process is adopted, that is, step b is replaced with the spray drying-thermal treatment process of steps d-e. The difference is only that in step b, spray pyrolysis is changed to post-treatment after spray drying. The spray conditions are: the inlet temperature is 280°C and the outlet temperature is 100°C; the heat treatment conditions are: sintering at 750°C for 6 h in an air atmosphere, and then used as a pretreatment precursor to participate in the subsequent steps c to e. Other operations and parameters are the same as in Example 1.

[0078] Comparative Example 5

[0079] Compared with Example 1, a two-stage spray pyrolysis process is adopted, that is, steps d and e are replaced with the spray pyrolysis process under the same conditions as step b. The difference is only that the spray drying and special treatment processes in steps d-e are replaced with spray pyrolysis. The conditions are: using nitrogen as the carrier gas, the temperature of the pyrolysis furnace is 700°C, and the residence time of the material in the pyrolysis furnace is 60 s. Other operations and parameters are the same as in Example 1.

[0080] The content of chlorine element in the obtained lithium iron phosphate material is tested by ICP; the BET of the material is tested by a specific surface area analyzer; the particle size and morphology distribution of the material are tested by SEM.

[0081] The material is mixed with PVDF and carbon black using NMP as a solvent to prepare a positive electrode sheet, and a button cell is assembled according to the standard of GB 31241-2014. Its electrochemical performance is tested. The test conditions are: using a Neware / Blue Power test system to test the electrochemical performance of the button cell (range I: 10 mA; range U: 5 V). The test voltage range is 2.5 - 4.5 V. The charge and discharge current for capacity test is set at 0.1C rate, the test temperature is room temperature 25°C, and the low-temperature test temperature is -40°C.

[0082] Table 1 shows the test data of the lithium iron phosphate products prepared in the examples and comparative examples

[0083]

[0084] Note: (a) The low-temperature performance refers to the low-temperature discharge performance: discharge capacity at -40°C at 0.1C / discharge capacity at room temperature (25°C) at 0.1C * 100%;

[0085] It can be seen from Table 1 that by using the double-spray process described in the present invention, materials with better low-temperature stability can be unexpectedly obtained.

Claims

1. A method for preparing lithium iron phosphate@C composite material by combined double spraying, characterized in that: The slurry of lithium source, iron source and phosphorus source is subjected to spray pyrolysis treatment to obtain a transformation activation precursor; wherein the atmosphere in the spray pyrolysis stage is an oxygen-containing atmosphere at a temperature of 600-900°C; The transformation activation precursor and the carbon source are composited into a slurry, spray dried, and heat treated to obtain a lithium iron phosphate@C composite material.

2. The method for preparing lithium iron phosphate@C composite material by combined double spraying as claimed in claim 1, characterized in that: The lithium source and iron source are water-soluble salts of the respective metal elements, preferably at least one of chloride, acetate, sulfate, and organic acid salt; Preferably, the phosphorus source is capable of ionizing PO4 3- The compound is preferably at least one of phosphoric acid, ammonium phosphate, diammonium hydrogen phosphate, and diammonium dihydrogen phosphate; Preferably, in the slurry, the molar ratio of Li, Fe and P is 1.01-1.06:1:1.01-1.04; Preferably, the solute in the slurry consists of a lithium source, an iron source and a phosphorus source; Preferably, the solvent in the slurry is water; Preferably, the concentration of iron in the slurry is 0.4-2.5 mol / L.

3. The method for preparing lithium iron phosphate@C composite material by combined double spraying as claimed in claim 1, characterized in that: The oxygen-containing atmosphere is at least one of oxygen, air, and a mixture of oxygen and protective gas; Preferably, the protective gas is at least one of nitrogen and argon; Preferably, the atomization pressure of spray pyrolysis is 0.5 to 3 MPa; Preferably, the spray pyrolysis time is 5 to 90 seconds.

4. The method for preparing lithium iron phosphate@C composite material by combined double spraying as claimed in claim 1, characterized in that: The carbon source is at least one of glucose, starch, sucrose, polyethylene glycol, polyvinyl alcohol, and cyclodextrin; Preferably, the weight ratio of the carbon source to the precursor is 0.1-0.3:

1.

5. The method for preparing lithium iron phosphate@C composite material by combined double spraying as claimed in claim 1, characterized in that: The inlet temperature of the spray drying stage is 200-300°C, and the outlet temperature is 70-120°C.

6. The method for preparing lithium iron phosphate@C composite material by combined double spraying as claimed in claim 1, characterized in that: The atmosphere during the heat treatment stage is at least one of nitrogen, argon, hydrogen, alkanes, and alkenes; Preferably, the temperature of the heat treatment stage is 650-800°C; Preferably, the heat treatment time is 4 to 24 hours.

7. A lithium iron phosphate@C composite material prepared by the method according to any one of claims 1 to 6; Preferably, it comprises spherical or quasi-spherical lithium iron phosphate primary particles and secondary particles formed by aggregation thereof, and an amorphous carbon layer coated on the surface of the primary particles and the secondary particles; The primary particle size is 200 to 800 nm, preferably 400 to 500 nm; The particle size of the secondary particles is 5 to 25 μm; The thickness of the amorphous carbon layer is less than 10 nm, preferably 2 to 10 nm, and more preferably 3 to 6 nm; Preferably, in lithium iron phosphate, the molar ratio of Fe / P is controlled at 0.96-0.99, and the molar ratio of Li / P is controlled at 1.01-1.06; the thickness of the amorphous carbon layer is 2-10 nm, and the amorphous carbon content is 1.0-2.5 wt.%, preferably 1.5-2 wt.%.

8. An application of a lithium iron phosphate@C composite material prepared by the method according to any one of claims 1 to 6, characterized in that: It is used as positive electrode active material to prepare lithium secondary batteries.

9. A positive electrode for a lithium secondary battery, comprising a current collector and a positive electrode material composited on the surface thereof, characterized in that: The positive electrode material comprises a lithium iron phosphate@C composite material prepared by the method according to any one of claims 1 to 6; Preferably, the positive electrode material further comprises a conductive agent and a binder; Preferably, in the positive electrode material, the content of the lithium iron phosphate@C composite material is above 60wt.%, preferably 75-95wt.%.

10. A lithium secondary battery, comprising a battery cell in which a positive electrode, a separator and a negative electrode are sequentially combined, characterized in that: The positive electrode is the positive electrode according to claim 9.

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