Lithium iron phosphate and preparation method therefor and use thereof, and ammonium salt compound and use thereof

Abstract

The present application relates to lithium iron phosphate and a preparation method therefor and a use thereof, and an ammonium salt compound and a use thereof. The preparation method for the lithium iron phosphate comprises the following steps: mixing iron phosphate, a lithium source, a carbon source, a dispersing aid, and a solvent to prepare a precursor slurry; and sintering the precursor slurry to prepare lithium iron phosphate; wherein the dispersing aid comprises an ammonium salt compound as shown in formula (1): wherein R1 is a carbon-containing organic group. The lithium iron phosphate prepared by the preparation method for the lithium iron phosphate can improve the charge / discharge capacity of a secondary battery.

Description

Lithium iron phosphate and its preparation method and application, ammonium salt compound and its application

[0001] Cross-references

[0002] This application claims priority from Chinese Patent Application No. 2023115234684, filed on November 15, 2023, entitled “Lithium iron phosphate, preparation method and application thereof, ammonium salt compound and application thereof”, which is incorporated herein by reference in its entirety. Technical Field

[0003] The present application relates to the technical field of secondary batteries, and in particular to lithium iron phosphate and a preparation method and application thereof, and an ammonium salt compound and application thereof. Background Art

[0004] Secondary batteries are increasingly widely used due to their clean and renewable characteristics. They mainly rely on the movement of active ions such as lithium ions between the positive and negative electrodes to generate electricity.

[0005] Lithium iron phosphate cathode materials have become one of the most widely used cathode materials due to their stable structure, excellent charge and discharge platform, high capacity, and good cycle performance. Traditional lithium iron phosphate is mainly prepared by the phosphorus iron process. The main process is: iron phosphate, lithium source, and carbon source are mixed in a solvent to prepare a slurry, and then sintered. During the mixing process of the traditional preparation process, the solid content of the slurry needs to be kept at a low level, because the high solid content increases the viscosity of the slurry, which is not conducive to the reaction to form lithium iron phosphate, resulting in an incomplete microstructure of the prepared lithium iron phosphate. Maintaining the solid content of the slurry at a low level greatly limits the production capacity of lithium iron phosphate. In addition, the traditional preparation process is accompanied by excessive side reactions. These factors will reduce the stability of lithium iron phosphate and have a negative impact on the battery performance, such as charge and discharge capacity, when used in battery preparation.

[0006] As people's demand for secondary batteries increases, traditional lithium iron phosphate positive electrode materials are becoming increasingly difficult to meet people's needs and need further improvement.

[0007] Summary of the Invention

[0008] Based on this, it is necessary to provide a lithium iron phosphate and its preparation method and application, an ammonium salt compound and its application, which are beneficial to improving the charge and discharge capacity of secondary batteries.

[0009] In a first aspect of the present application, a method for preparing lithium iron phosphate is provided, comprising the following steps:

[0010] The iron phosphate, lithium source, carbon source, dispersing aid and solvent are mixed to prepare a precursor slurry;

[0011] sintering the precursor slurry to prepare lithium iron phosphate;

[0012] Wherein, the dispersing aid includes an ammonium salt compound as shown in formula (1):

[0013] R1 is a carbon-containing organic group.

[0014] In the preparation method of the above-mentioned lithium iron phosphate preparation method, iron phosphate, lithium source, carbon source, dispersing aid and solvent are first mixed and treated. The dispersing aid includes an ammonium salt compound with a specific structure. After ionization, the ammonium salt compound will form a negatively charged ion group. The negatively charged end is adsorbed on the surface of the iron phosphate component in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form a steric hindrance. The steric hindrance reduces the probability of agglomeration between solid particles in the precursor slurry, which is beneficial to the reaction of components in the subsequent sintering treatment, improves the microstructural integrity of the obtained lithium iron phosphate, and the obtained lithium iron phosphate particles have good morphology, a smoother surface and a stable structure; and the ammonium salt compound will decompose into easily vaporized products during the sintering process, reducing the by-products in the lithium iron phosphate and improving the charge and discharge capacity of the secondary battery.

[0015] In the above-mentioned preparation method of lithium iron phosphate, the ammonium salt compound with a specific structure can play an excellent dispersing role. Even if the precursor slurry has a high solid content, lithium iron phosphate with good particle morphology and complete microstructure can be obtained.

[0016] In some embodiments, the ammonium salt compound includes at least one of the compounds represented by formula (1A) to formula (1B):

[0017] Among them, R 11 ~R 13 are independently selected from H, substituted or unsubstituted alkyl having 1 to 5 carbon atoms, hydroxyl or -C(O)O - NH4 + ;

[0018] Each R2 and each R3 are independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0019] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH4 + Any of substituted alkanediyl groups having 1 to 5 carbon atoms.

[0020] The use of compounds of formula (1A) and formula (1B) as dispersing aids can improve the microstructural stability of the obtained lithium iron phosphate; in particular, the compound of formula (1B) also contains a specific phosphate ester structure and is relatively active. While playing a dispersing role, it can also promote the reaction between iron phosphate, lithium source and carbon source, promote the formation of lithium iron phosphate, and further improve the microstructural stability of the obtained lithium iron phosphate.

[0021] Moreover, since the compound of formula (1B) promotes the reaction between iron phosphate, lithium source and carbon source, the temperature of the subsequent sintering treatment can be reduced, and lithium iron phosphate with a perfect microstructure can be obtained at a lower temperature. The lower sintering temperature can also reduce the by-product reactions in the sintering process, reduce the content of by-product impurities such as elemental iron and iron phosphide in the obtained lithium iron phosphate, and further improve the charge and discharge capacity of the secondary battery.

[0022] In some embodiments, the compound represented by formula (1A) satisfies at least one of the following conditions (1) to (2):

[0023] (1)R 11 ~R 13 Each independently selected from H, an unsubstituted alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with a hydroxyl group, a hydroxyl group, or -C(O)O - NH4 + ;

[0024] (2)R 11 ~R 13 At least one of them is selected from -C(O)O - NH4 + .

[0025] -C(O)O in the compound represented by molecular formula (1A) - NH4 + The more groups there are, the stronger the dispersing ability is.

[0026] In some embodiments, the ammonium salt compound includes at least one of ammonium citrate, diamine citrate, and compounds represented by formula (b-1) to (b-2):

[0027] Among them, R 21 Selected from alkyl groups having 1 to 3 carbon atoms or -C(O)O - NH4 + .

[0028] In some embodiments, the mass of the dispersing aid accounts for 0.3% to 7% of the mass of the ferric phosphate;

[0029] Optionally, the mass of the dispersing aid accounts for 0.5% to 1% of the mass of the ferric phosphate.

[0030] By regulating the dosage of the dispersing agent, the microstructural stability of the prepared lithium iron phosphate can be further improved.

[0031] The study found that within a certain range, the greater the percentage (W1%) of the mass of the dispersing agent to the mass of the iron phosphate, the more complete the microstructure of the resulting lithium iron phosphate and the higher the charge and discharge capacity of the secondary battery. When W1% reaches around 1%, the growth rate of the secondary battery's charge and discharge capacity essentially reaches equilibrium. Further increases in W1% lead to a smaller growth rate, and when the charge and discharge capacity increases to a level that causes significant lithium deposition, it negatively impacts the battery's lifespan. Therefore, the range of W1% can be further controlled to further increase the charge and discharge capacity of the secondary battery while reducing the probability of lithium deposition.

[0032] In some embodiments, the sintering temperature is 780° C. to 820° C.;

[0033] Optionally, the sintering temperature is 750°C to 790°C.

[0034] In some embodiments, the preparation method satisfies at least one of the following conditions (1) to (2):

[0035] (1) The carbon source comprises at least one of glucose, sucrose, starch, phenolic resin, cyclodextrin, polyethylene, polyethylene glycol and polyvinyl alcohol;

[0036] (2) The lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

[0037] In some embodiments, the precursor slurry satisfies at least one of the following conditions (1) to (3):

[0038] (1) The solid content of the precursor slurry is ≥45%;

[0039] (2) The solvent includes at least one of water and an alcoholic organic solvent.

[0040] In a second aspect of the present application, a lithium iron phosphate is provided, wherein the lithium iron phosphate is prepared by the preparation method of the lithium iron phosphate according to the first aspect.

[0041] In the preparation method of the above-mentioned lithium iron phosphate preparation method, the dispersing aid includes an ammonium salt compound with a specific structure. After ionization, the ammonium salt compound will form a negatively charged ion group. The negatively charged end is adsorbed on the surface of the iron phosphate component in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form a steric hindrance. The steric hindrance reduces the probability of agglomeration between solid particles in the precursor slurry, which is beneficial to the reaction of components in the subsequent sintering treatment, improves the microstructural integrity of the obtained lithium iron phosphate, and the obtained lithium iron phosphate particles have good morphology and stable structure; and the ammonium salt compound will decompose into easily vaporized products during the sintering process, reducing the by-products in the lithium iron phosphate and improving the charge and discharge capacity of the secondary battery.

[0042] In a third aspect of the present application, an ammonium salt compound is provided, wherein the ammonium salt compound is as shown in formula (1B):

[0043] Each R2 and each R3 are independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0044] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH4 + Any of substituted alkanediyl groups having 1 to 5 carbon atoms.

[0045] The fifth aspect of the present application provides the use of the ammonium salt compound of the third aspect, wherein the ammonium salt compound is used as a dispersing aid.

[0046] The fifth aspect of the present application provides the use of the ammonium salt compound of the third aspect in the preparation of lithium iron phosphate.

[0047] The compound of formula (1B) also contains a specific phosphate group structure with relatively high activity. When used to prepare lithium iron phosphate, it not only plays a dispersing role, but also promotes the reaction between iron phosphate, lithium source and carbon source, promotes the formation of lithium iron phosphate, and further improves the microstructural stability of the prepared lithium iron phosphate.

[0048] Moreover, since the compound of formula (1B) promotes the reaction between iron phosphate, lithium source and carbon source, the temperature of the subsequent sintering treatment can be reduced, and lithium iron phosphate with a perfect microstructure can be obtained at a lower temperature. The lower sintering temperature can also reduce the by-product reactions in the sintering process, reduce the by-product impurities such as elemental iron and iron phosphide in the obtained lithium iron phosphate, and further improve the charge and discharge capacity of the secondary battery.

[0049] In a sixth aspect of the present application, a positive electrode sheet is provided, comprising a current collector and a positive electrode active layer disposed on a surface of the current collector, wherein a component of the positive electrode active layer comprises the lithium iron phosphate of the second aspect.

[0050] In a seventh aspect of the present application, a secondary battery is provided, wherein the secondary battery comprises the lithium iron phosphate of the second aspect or the positive electrode sheet of the sixth aspect.

[0051] In an eighth aspect of the present application, an electrical device is provided, comprising the secondary battery according to the seventh aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Various other advantages and benefits will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiment below. The accompanying drawings are for illustration purposes only and are not to be considered as limiting the present application. The same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0053] FIG1 is a schematic diagram of the mechanism of action of a dispersing aid during the preparation of lithium iron phosphate in one embodiment;

[0054] FIG2 is a schematic diagram of an embodiment of a battery cell;

[0055] FIG3 is an exploded view of FIG2 ;

[0056] FIG4 is a schematic diagram of an embodiment of a battery pack;

[0057] FIG5 is an exploded view of FIG4;

[0058] 6 is a schematic diagram of an embodiment of an electric device using a secondary battery as a power source;

[0059] FIG7 is a thermogravimetric curve of the solid material after the precursor slurry is dried in Example 1;

[0060] FIG8 is a comparison of infrared curves obtained by Fourier transform infrared (FTIR) testing of the lithium iron phosphate prepared in Example 1, the solid material after drying the precursor slurry, and the ammonium salt compound;

[0061] FIG9 is a scanning electron microscope image of the lithium iron phosphate prepared in Example 1;

[0062] FIG10 is a scanning electron microscope image of the lithium iron phosphate prepared in Example 5;

[0063] FIG11 is a scanning electron microscope image of the lithium iron phosphate prepared in Example 6;

[0064] FIG12 is a scanning electron microscope image of the lithium iron phosphate prepared in Comparative Example 1.

[0065] Description of reference numerals:

[0066] 1. Battery pack; 2. Upper box; 3. Lower box; 4. Battery cell; 41. Shell; 42. Electrode assembly; 43. Cover; 5. Electrical device. DETAILED DESCRIPTION

[0067] To make the above-mentioned objects, features, and advantages of the present application more clearly understood, the specific embodiments of the present application are described in detail below. The following description sets forth many specific details to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways than those described herein, and those skilled in the art can make similar modifications without violating the scope of the present application. Therefore, the present application is not limited to the specific embodiments disclosed below.

[0068] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of the technical features being referred to. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of such features. Throughout the description of this application, "plurality" means at least two, for example, two, three, etc., unless otherwise specifically defined.

[0069] In this application, the term "alkyl" refers to a group formed when an alkane loses a hydrogen, such as methane loses a hydrogen to form a methyl group; "alkenyl or alkynyl" refers to a group formed when an alkene or alkyne loses a hydrogen, such as ethylene loses a hydrogen to form vinyl, and acetylene loses a hydrogen to form ethynyl.

[0070] In the present application, the number of carbon atoms in the "alkyl group having 1 to 5 carbon atoms" may be 1 to 5, including 1, 2, 3, 4, and 5. Non-limiting examples include methane, ethyl, and n-propyl.

[0071] In the present application, the term "substituted or unsubstituted" means that the functional group described after the term may or may not have a substituent.

[0072] Traditionally, lithium iron phosphate is produced using a ferrophosphorus process. The main process involves mixing iron phosphate, a lithium source, and a carbon source in a solvent to create a slurry, followed by sintering. To increase the solids content of the slurry and thus improve production capacity, a dispersant is often added during the mixing process to reduce the chance of slurry agglomeration.

[0073] Some technologies use traditional polymer dispersants such as polyvinyl pyrrolidone (PVP) and polyvinyl alcohol (PVA) to prepare lithium iron phosphate. However, since these polymer dispersants will gel and freeze at room temperature, the viscosity of the slurry will increase and it will need to be heated to flow, making it unsuitable for industrial production.

[0074] Other technologies use alkali metal salts of small molecule organic acids as dispersants, which can reduce the viscosity of the slurry and increase the solid content of the slurry. Further experimental studies have found that: using alkali metal salts of organic acids as dispersants, the Na + , K +The plasma is not easy to dissipate during the sintering stage, and most of it remains in the lithium iron phosphate, resulting in an increase in the content of alkali metal elements in the lithium iron phosphate. When used to prepare batteries, it will have a negative impact on the performance of the battery.

[0075] Thus, after a lot of exploration, the lithium iron phosphate and its preparation method in this application were obtained.

[0076] In one embodiment of the present application, a method for preparing lithium iron phosphate is provided, comprising the following steps S10 to S20.

[0077] Step S10: mixing iron phosphate, a lithium source, a carbon source, a dispersing aid and a solvent to prepare a precursor slurry.

[0078] Step S20: sintering the precursor slurry to prepare lithium iron phosphate.

[0079] The dispersing aid includes an ammonium salt compound as shown in formula (1):

[0080] R1 is a carbon-containing organic group.

[0081] In the preparation method of the above-mentioned lithium iron phosphate preparation method, iron phosphate, lithium source, carbon source, dispersing aid and solvent are first mixed and treated. The dispersing aid includes an ammonium salt compound with a specific structure. After ionization, the ammonium salt compound will form a negatively charged ion group. The negatively charged end is adsorbed on the surface of the iron phosphate component in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form a steric hindrance. The steric hindrance reduces the probability of agglomeration between solid particles in the precursor slurry, which is beneficial to the reaction of components in the subsequent sintering treatment, improves the microstructural integrity of the obtained lithium iron phosphate, and the obtained lithium iron phosphate particles have good morphology and stable structure; and the ammonium salt compound will decompose into easily vaporized products during the sintering process, reducing the by-products in the lithium iron phosphate and improving the charge and discharge capacity of the secondary battery.

[0082] In the above-mentioned preparation method of lithium iron phosphate, the ammonium salt compound with a specific structure can play an excellent dispersing role, and even if the precursor slurry has a high solid content, lithium iron phosphate with good particle morphology and complete microstructure can be obtained.

[0083] Please refer to Figure 1, which is a schematic diagram of the action mechanism of the dispersing aid in the preparation process of lithium iron phosphate in one embodiment, wherein Z1 represents iron phosphate, Z2 represents an ammonium salt compound, the negatively charged end is adsorbed on the surface of the iron phosphate component in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form steric hindrance.

[0084] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-5000.

[0085] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-1000.

[0086] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-500.

[0087] "Relative molecular mass" refers to the sum of the relative atomic masses (Ar) of the individual atoms in a chemical formula.

[0088] In some embodiments, the ammonium salt compound includes at least one of the compounds represented by formula (1A) to formula (1B):

[0089] Among them, R 11 ~R 13 are independently selected from H, substituted or unsubstituted alkyl having 1 to 5 carbon atoms, hydroxyl or -C(O)O - NH4 + .

[0090] Each R2 and each R3 are independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0091] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH4 + Any of substituted alkanediyl groups having 1 to 5 carbon atoms.

[0092] The use of compounds of formula (1A) and formula (1B) as dispersing aids can improve the microstructural stability of the obtained lithium iron phosphate; in particular, the compound of formula (1B) also contains a specific phosphate ester structure and is relatively active. While playing a dispersing role, it can also promote the reaction between iron phosphate, lithium source and carbon source, promote the formation of lithium iron phosphate, and further improve the microstructural stability of the obtained lithium iron phosphate.

[0093] Moreover, since the compound of formula (1B) promotes the reaction between iron phosphate, lithium source and carbon source, the temperature of the subsequent sintering treatment can be reduced, and lithium iron phosphate with a perfect microstructure can be obtained at a lower temperature. The lower sintering temperature can also reduce the by-product reactions in the sintering process, reduce the by-product impurities such as elemental iron and iron phosphide in the obtained lithium iron phosphate, and further improve the charge and discharge capacity of the secondary battery.

[0094] In some embodiments, R 11 ~R 13 Each independently selected from H, an unsubstituted alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with a hydroxyl group, a hydroxyl group, or -C(O)O - NH4 + .

[0095] In some embodiments, R 11 ~R 13 Each of the following groups is independently selected from H, an unsubstituted alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with a hydroxyl group, a hydroxyl group, or -C(O)O - NH4 + .

[0096] In some embodiments, R 11 ~R 13 Each of the following groups is independently selected from H, an unsubstituted alkyl group having 1 to 3 carbon atoms, an alkyl group having 1 to 3 carbon atoms substituted with a hydroxyl group, a hydroxyl group, or -C(O)O - NH4 + .

[0097] In some embodiments, R 11 ~R 13 are independently selected from H, methyl, ethyl, propyl, hydroxyl or -C(O)O - NH4 + .

[0098] In some embodiments, R 11 ~R 13 At least one of them is selected from -C(O)O - NH4 + .

[0099] -C(O)O in the compound represented by molecular formula (1A) - NH4 + The more groups there are, the stronger the dispersing ability is.

[0100] In some embodiments, R 11 ~R 13 At least two of them are selected from -C(O)O - NH4 + .

[0101] In some embodiments, each R2 and each R3 are independently selected from any one of H and an unsubstituted alkyl group having 1 to 5 carbon atoms.

[0102] In some embodiments, each R2 and each R3 are independently selected from any one of H and an unsubstituted alkyl group having 1 to 5 carbon atoms.

[0103] In some embodiments, each R2 and each R3 are independently selected from any one of H and an unsubstituted alkyl group having 1 to 3 carbon atoms.

[0104] In some embodiments, each R2 is the same.

[0105] In some embodiments, each R3 is the same.

[0106] In some embodiments, R2 and R3 are the same.

[0107] In some embodiments, L is selected from a single bond or -C(O)O - NH4 + The substituted alkane substituent has 1 to 5 carbon atoms.

[0108] In some embodiments, L is selected from a single bond or -C(O)O - NH4 + The substituted alkane substituent has 1 to 3 carbon atoms.

[0109] The preparation method of the compound of formula (1B) is as follows:

[0110] Compound (a) and diammonium hydrogen phosphate are mixed and reacted to prepare a compound of formula (1B).

[0111] The structure of diammonium hydrogen phosphate is as follows:

[0112] The structure of compound (a) is as follows:

[0113] In some embodiments, during the mixing reaction, the pH value of the reaction solution is maintained at 5.0-7.4.

[0114] In some embodiments, the mixing reaction time is 4 hours to 24 hours.

[0115] Condensation and dehydration reactions occur during the mixing reaction, including condensation esterification and condensation to etherification reactions.

[0116] The types of L, each R2 and each R3 are selected as above and will not be described in detail here.

[0117] In some embodiments, the ammonium salt compound includes at least one of the compounds represented by formula (2A) to formula (2B):

[0118] In some embodiments, the ammonium salt compound includes at least one of ammonium citrate, diamine citrate, and compounds of formula (b-1) to (b-2):

[0119] Among them, R 21 Selected from alkyl groups having 1 to 3 carbon atoms or -C(O)O - NH4 + .

[0120] In some embodiments, R21 Selected from -C(O)O - NH4 + .

[0121] In some embodiments, the mass of the dispersing aid accounts for 0.3% to 7% of the mass of the ferric phosphate.

[0122] In some embodiments, the mass of the dispersing aid accounts for 0.5% to 1% of the mass of the ferric phosphate.

[0123] By regulating the amount of the dispersing agent, the microstructural stability of the prepared lithium iron phosphate is further improved, and the sphericity is high.

[0124] In the above “0.3% to 7%”, the values ​​include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include but are not limited to the point values ​​in the embodiments and: 0.3%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.3%, 1.5%, 1.7%, 1.9%, 2%, 2.1%, 2.3%, 2.5%, 2.7%, 2.9%, 3% , 3.1%, 3.3%, 3.5%, 3.7%, 3.9%, 4%, 4.1%, 4.3%, 4.5%, 4.7%, 4.9%, 5%, 5.1%, 5.3%, 5.5%, 5.7%, 5.9%, 6%, 6.1%, 6.3%, 6.5%, 6.7%, 6.9%, 7%; or a range consisting of any two point values, for example: 0.3%~7%, 0.3%~6%, 0.3%~5%, 0.3%~4%, 0.3%~3%, 0.3%~2%, 0.3%~1%, 0.3%~0.5%, 0.5%~7%, 0.5%~6%, 0.5%~5%, 0.5%~4%, 0.5%~3%, 0.5%~2%, 0.5%~1%, 1%~7%, 1%~6%, 1%~5%, 1%~4%, 1%~3%, 1%~2%.

[0125] In some embodiments, the sintering temperature is 780°C to 820°C.

[0126] Optionally, the sintering temperature is 750°C to 790°C.

[0127] The use of the ammonium salt compound with a specific structure in the present application as a dispersing aid can promote the formation of lithium iron phosphate, and lithium iron phosphate with a perfect microstructure can be obtained at a lower temperature. The lower sintering temperature can also reduce the by-product reactions in the sintering process, reduce the by-product impurities such as elemental iron and iron phosphide in the obtained lithium iron phosphate, and further improve the charge and discharge capacity of the secondary battery.

[0128] In the above "780℃~820℃", the values ​​include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Specific examples include but are not limited to the point values ​​in the embodiments and: 780℃, 790℃, 800℃, 810℃, 820℃; or a range consisting of any two point values.

[0129] In some embodiments, the sintering time is 19 hours to 21 hours.

[0130] In some embodiments, in the mixing process of step S10, the specific steps are as follows:

[0131] mixing a dispersing aid and a solvent to form a mixed mixture;

[0132] The iron phosphate, the lithium source, the carbon source and the mixed mixture are mixed to prepare a slurry.

[0133] In some embodiments, the mixing process is performed under stirring conditions for 0.5 h to 1 h.

[0134] In some embodiments, before the sintering step, the method further includes spray drying the precursor slurry.

[0135] After spray drying, a solid precursor in the form of spherical particles can be obtained.

[0136] In some embodiments, the spray drying temperature is 280°C to 350°C.

[0137] In some embodiments, the ratio of the molar number of lithium in the lithium source to the molar number of phosphorus and the molar number of iron in the iron phosphate conforms to the chemical composition of lithium iron phosphate (LiFePO 4 ).

[0138] In some embodiments, the mass of the lithium source accounts for 0.235% to 0.25% of the mass of the iron phosphate.

[0139] In some embodiments, the mass of the lithium source accounts for 0.237% to 0.25% of the mass of the iron phosphate.

[0140] In some embodiments, the mass of the carbon source accounts for 5% to 15% of the mass of the ferric phosphate.

[0141] In some embodiments, the mass of the carbon source accounts for 5% to 10% of the mass of the ferric phosphate.

[0142] In some embodiments, the mass of the carbon source accounts for 5% to 9% of the mass of the ferric phosphate.

[0143] In some embodiments, the mass of the carbon source accounts for 6% to 9% of the mass of the ferric phosphate.

[0144] In some embodiments, the carbon source comprises at least one of glucose, sucrose, starch, phenolic resin, cyclodextrin, polyethylene, polyethylene glycol, and polyvinyl alcohol.

[0145] In some embodiments, the lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate, and lithium acetate.

[0146] In some embodiments, the solid content of the precursor slurry is greater than or equal to 45%.

[0147] In some embodiments, the solid content of the precursor slurry is 45% to 55%.

[0148] In the above-mentioned preparation method of lithium iron phosphate, the ammonium salt compound with a specific structure can play an excellent dispersing role. Even if the precursor slurry has a high solid content, lithium iron phosphate with good particle morphology and complete microstructure can be obtained, which greatly improves production capacity.

[0149] In some embodiments, the solvent includes at least one of water and an alcoholic organic solvent.

[0150] In some embodiments, the solvent includes water.

[0151] In one embodiment of the present application, a lithium iron phosphate is further provided. The lithium iron phosphate is prepared using the above-mentioned method for preparing lithium iron phosphate.

[0152] In the preparation method of the above-mentioned lithium iron phosphate preparation method, the dispersing aid includes an ammonium salt compound with a specific structure. After ionization, the ammonium salt compound will form a negatively charged ion group. The negatively charged end is adsorbed on the surface of the iron phosphate component in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form a steric hindrance. The steric hindrance reduces the probability of agglomeration between solid particles in the precursor slurry, which is beneficial to the reaction of components in the subsequent sintering treatment, improves the microstructural integrity of the obtained lithium iron phosphate, and the obtained lithium iron phosphate particles have good morphology and stable structure; and the ammonium salt compound will decompose into easily vaporized products during the sintering process, reducing the by-products in the lithium iron phosphate and improving the charge and discharge capacity of the secondary battery.

[0153] In one embodiment of the present application, an ammonium salt compound is provided. The ammonium salt compound is shown in formula (1B):

[0154] Each R2 and each R3 is independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.

[0155] The types of R2 and R3 are the same as those described above and will not be described again here.

[0156] The preparation method of the ammonium salt compound represented by formula (1B) is the same as described above and will not be repeated here.

[0157] The present application also provides the use of the above-mentioned ammonium salt compound in the preparation of lithium iron phosphate.

[0158] The compound of formula (1B) also contains a specific phosphate group structure with relatively high activity. When used to prepare lithium iron phosphate, it not only plays a dispersing role, but also promotes the reaction between iron phosphate, lithium source and carbon source, promotes the formation of lithium iron phosphate, and further improves the microstructural stability of the prepared lithium iron phosphate.

[0159] Moreover, since the compound of formula (1B) promotes the reaction between iron phosphate, lithium source and carbon source, the temperature of the subsequent sintering treatment can be reduced, and lithium iron phosphate with a perfect microstructure can be obtained at a lower temperature. The lower sintering temperature can also reduce the by-product reactions in the sintering process, reduce the by-product impurities such as elemental iron and iron phosphide in the obtained lithium iron phosphate, and further improve the charge and discharge capacity of the secondary battery.

[0160] In some embodiments, the above-mentioned ammonium salt compound is used as a dispersing aid.

[0161] One embodiment of the present application further provides a positive electrode sheet, which includes a current collector and a positive electrode active layer disposed on a surface of the current collector, wherein the component of the positive electrode active layer includes the above-mentioned lithium iron phosphate.

[0162] In some embodiments, based on the total weight of the positive electrode active layer, the weight ratio of the lithium iron phosphate material in the positive electrode active layer is 80 wt % to 100 wt %.

[0163] In any embodiment of the present application, the components of the positive electrode active layer further include a positive electrode conductor and a positive electrode binder.

[0164] The positive electrode conductive agent may be any commonly used conductive agent in the art, including but not limited to at least one of graphite, carbon nanotubes, nanofibers, carbon black, and graphene. Specifically, the positive electrode conductive agent may be selected from at least one of SP, KS-6, acetylene black, branched Ketjen black ECP, SFG-6, vapor-grown carbon fiber (VGCF), carbon nanotubes (CNTs), graphene, and composite conductive agents thereof.

[0165] The weight ratio of the positive electrode conductive agent in the positive electrode active layer is 0 to 20 wt % based on the total weight of the positive electrode active layer.

[0166] In any embodiment of the present application, the binder of the above-mentioned positive electrode binder can be polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, hydrogenated nitrile rubber, styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS) and at least one of fluorine-containing acrylate resin.

[0167] The weight ratio of the positive electrode binder in the positive electrode active layer is 0 to 30 wt % based on the total weight of the positive electrode active layer.

[0168] In any embodiment of the present application, a positive electrode sheet can be prepared by the following method: dispersing the above-mentioned components for preparing a positive electrode sheet in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; applying the positive electrode slurry to a current collector, and performing drying, cold pressing, and other processes to obtain a positive electrode sheet. The positive electrode slurry has a solid content of 40 wt% to 80 wt%, and a viscosity at room temperature adjusted to 5000 mPa·s to 25000 mPa·s. The positive electrode slurry is applied to the surface of the positive electrode current collector, dried, and then cold-pressed on a cold rolling mill to form a positive electrode sheet.

[0169] In some embodiments, the compacted density of the positive electrode sheet is 3.0 g / cm 3 ~3.6g / cm 3 , optional 3.3g / cm 3 ~3.5g / cm 3 The calculation formula for compacted density is:

[0170] Compaction density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0171] In one embodiment of the present application, a secondary battery is provided. The battery includes the lithium iron phosphate or positive electrode sheet as described above.

[0172] The secondary battery further includes a negative electrode sheet, a separator, and an electrolyte. The negative electrode sheet, the separator, and the electrolyte are exemplified below, including but not limited to the following.

[0173] Electrolyte: Generally, the electrolyte includes electrolyte salt and solvent.

[0174] In some embodiments, the electrolyte salt may be selected from electrolyte salts commonly used in the art, such as lithium ion electrolyte salts.

[0175] As an example, the lithium ion electrolyte salt includes, but is not limited to, one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0176] In some embodiments, the solvent can be selected from one or more of fluoroethylene carbonate (FEC), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS) and diethyl sulfone (ESE).

[0177] In some embodiments, the concentration of the electrolyte salt in the electrolyte solution is generally 0.5 mol / L to 15 mol / L.

[0178] In some embodiments, the electrolyte may optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, and additives that improve battery high or low temperature performance.

[0179] Separator: The separator is placed between the positive electrode and the negative electrode.

[0180] The type of the isolation membrane of the present application can be any known porous structure isolation membrane with good chemical stability and mechanical stability.

[0181] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film. When the separator is a multi-layer composite film, the materials of each layer can be the same or different.

[0182] The thickness of the diaphragm is controlled to be 2 μm to 15 μm; optionally, the thickness of the diaphragm is controlled to be 2 μm to 13 μm.

[0183] Negative electrode sheet: The negative electrode sheet can be a negative electrode sheet used in various secondary battery systems in the field.

[0184] In some embodiments, the secondary battery is a lithium metal secondary battery, and the negative electrode sheet can be a negative electrode sheet that can be used for lithium metal batteries and is well known in the art.

[0185] In some embodiments, the negative electrode sheet directly adopts a lithium-containing metal sheet.

[0186] In another embodiment, the negative electrode sheet includes a lithium-containing metal layer and a conductive layer stacked together.

[0187] Furthermore, the lithium-containing metal in the lithium-containing metal sheet and the lithium-containing metal layer can be lithium metal, or an alloy formed by lithium metal and other metal or non-metal elements.

[0188] Further, the other metals include at least one of tin (Sn), zinc (Zn), aluminum (Al), magnesium (Mg), silver (Ag), gold (Au), gallium (Ga), indium (In) and platinum (Pt); the non-metallic elements include at least one of boron (B), carbon (C) and silicon (Si).

[0189] In some embodiments, the conductive layer may be copper foil.

[0190] In any embodiment of the present application, the negative electrode sheet can be prepared by directly pressing a lithium-containing metal sheet to obtain the negative electrode sheet, or by stacking and pressing the lithium-containing metal layer and the conductive layer to obtain the negative electrode sheet.

[0191] In some embodiments, the secondary battery is a lithium-ion secondary battery, and the negative electrode sheet can be a negative electrode sheet that can be used for lithium-ion batteries and is well known in the art.

[0192] In some embodiments, the negative electrode sheet includes a current collector and a negative electrode active layer supported on a surface of the current collector.

[0193] The components of the negative electrode active layer include a negative electrode active material.

[0194] The negative electrode active material may be any commonly used negative electrode active material in this application.

[0195] In any embodiment of the present application, the above-mentioned negative electrode active material includes at least one of mesocarbon microbeads, graphite, glassy carbon, carbon nanotubes, carbon-carbon composite materials, carbon fibers, hard carbon, soft carbon, silicon-based materials, tin-based materials, magnesium-based materials and iron-based materials.

[0196] Optionally, specific examples of the above-mentioned negative electrode active material include, but are not limited to: at least one of mesocarbon microbeads, natural graphite, artificial graphite, graphene, glassy carbon, carbon nanotubes, carbon fibers, hard carbon, soft carbon, iron oxide, tin oxide, silicon oxide, magnesium oxide, silicon-carbon composites, lithium metal, and lithium metal alloys.

[0197] In any embodiment of the present application, the mass proportion of the negative electrode active material in the negative electrode active layer is 70% to 100%.

[0198] In any embodiment of the present application, the components of the negative electrode active layer further include a negative electrode conductive agent and a negative electrode binder.

[0199] In any embodiment of the present application, the above-mentioned negative electrode conductive agent can be a conductive material commonly used in the art, including but not limited to: at least one of graphite, carbon nanotubes, nanofibers, carbon black and graphene. Specifically, it can include at least one of conductive carbon black (super pll, referred to as SP), conductive graphite SFG-6, conductive graphite KS-6, acetylene black, branched superconducting carbon black Ketjen black (ECP), vapor grown carbon fiber (VGCF), carbon nanotubes (CNTs) and graphene and composite conductive agents thereof.

[0200] The weight ratio of the negative electrode conductive agent in the negative electrode active layer is 0 to 20 wt % based on the total weight of the negative electrode active layer.

[0201] The negative electrode binder can be a binder commonly used in the art, and can be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA) and carboxymethyl chitosan (CMCS).

[0202] The weight ratio of the negative electrode binder in the negative electrode active layer is 0 to 30 wt % based on the total weight of the negative electrode active layer.

[0203] In any embodiment of the present application, the negative electrode active layer may further optionally include other additives, such as a thickener, such as sodium carboxymethyl cellulose (CMC-Na), etc. Based on the total weight of the negative electrode active layer, the weight ratio of the other additives in the negative electrode active layer is 0 to 15 wt%.

[0204] In any embodiment of the present application, the current collector in the negative electrode sheet may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil.

[0205] The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material on a polymer substrate.

[0206] In some embodiments, the metal material is selected from any one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy.

[0207] In some embodiments, the polymer material substrate includes at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0208] In any embodiment of the present application, the negative electrode sheet can be prepared by the following method: the above-mentioned components for preparing the negative electrode sheet, such as the negative electrode active material, the conductive agent, the binder and any other components are dispersed in a solvent to form a negative electrode slurry; the negative electrode slurry is coated on the negative electrode current collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0209] In some embodiments, the solvent includes but is not limited to water.

[0210] In some embodiments, the solid content of the negative electrode slurry is 30 wt % to 70 wt %, and the viscosity at 25° C. is adjusted to 2000 mPa·s to 10000 mPa·s.

[0211] In some embodiments, the surface density of the negative electrode active material contained in the negative electrode sheet is 0.005 g / cm 2 ~0.03g / cm 2 .

[0212] The area density of the negative electrode active material = the mass of the negative electrode active material / the area of ​​the negative electrode sheet.

[0213] The secondary battery of the present application may be in a cylindrical, square, or any other shape. For example, FIG2 shows a battery cell 4 of a square structure as an example.

[0214] In some embodiments, referring to FIG3 , a battery cell 4 includes a housing and an electrode assembly 42 . The housing may include a shell 41 and a cover plate 43 . Shell 41 may include a bottom plate and side plates connected to the bottom plate, which together form a receiving cavity. Shell 41 has an opening communicating with the receiving cavity, and cover plate 43 may be positioned over the opening to seal the receiving cavity.

[0215] The positive electrode sheet, negative electrode sheet, and separator can be wound or laminated to form an electrode assembly 42. The electrode assembly 42 is enclosed in a housing. Electrolyte is infiltrated into the electrode assembly 42. The number of electrode assemblies 42 included in a battery cell 4 can be one or more, and can be adjusted based on demand.

[0216] The present application also provides an electrical device, which includes the above-mentioned secondary battery.

[0217] Furthermore, in the above-mentioned electrical device, the secondary battery may exist in the form of a battery cell, or may be further assembled into a battery pack.

[0218] Figures 3 and 4 illustrate an exemplary battery pack 1. Battery pack 1 includes a battery case and one or more battery cells 4 disposed within the battery case. The battery case comprises an upper case 2 and a lower case 3. The upper case 2 can be placed over the lower case 3 to form an enclosed space for the battery cells 4.

[0219] The plurality of battery cells 4 can be arranged in the battery box in any manner.

[0220] The secondary battery or the battery pack assembled therefrom can be used as a power source for an electrical device or as an energy storage unit for an electrical device.

[0221] The above-mentioned electrical devices may be, but are not limited to, mobile devices, electric vehicles, electric trains, ships, satellites, energy storage systems, etc.

[0222] In some embodiments, the mobile device may be a mobile phone or a laptop computer, etc.

[0223] In some embodiments, electric vehicles include, but are not limited to, pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, and the like.

[0224] Figure 5 shows an example of an electric device 5. The electric device 5 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of the electric device 5, a battery pack can be used.

[0225] As another example, the power-consuming device may be a mobile phone, a tablet computer, a laptop computer, etc. Such a device is usually required to be light and thin, and may use a battery as a power source.

[0226] The present application will be described below in conjunction with specific embodiments, but the present application is not limited to the following embodiments. It should be understood that the attached claims summarize the scope of the present application. Under the guidance of the concept of the present application, those skilled in the art should realize that certain changes made to the various embodiments of the present application will be covered by the spirit and scope of the claims of the present application.

[0227] Example 1

[0228] S1: Preparation of lithium iron phosphate

[0229] (1) An ammonium salt compound (ammonium citrate) is dissolved in deionized water to obtain a mixture.

[0230] (2) Add ferric phosphate, lithium carbonate, and glucose to a stirring tank. While stirring with a paddle, add the mixture to the stirring tank and stir and grind for 20 minutes to mix evenly to obtain a precursor slurry. The mass of the ammonium salt compound in the mixture is W1% of the mass of the ferric phosphate, the mass of the lithium carbonate is W2% of the mass of the ferric phosphate, and the mass of the glucose is W3% of the mass of the ferric phosphate. See Table 1 for details.

[0231] S2: Solid content test

[0232] (1) Sample weighing: Weigh a certain amount of sample (10 g), record the accuracy and resolution of the weighing instrument, and record the sample weight as M0.

[0233] (2) Drying: Adjust the set temperature to 100℃, place the sample in an oven and dry it until the mass no longer changes. Record the height of the load rod within a fixed time, and record the weight after drying as M1.

[0234] (3) Cooling: Cool the measuring cup to room temperature, remove the condensed water and then weigh it.

[0235] (4) Calculate the solid content: Solid content = M1 / M0 × 100%. See Table 1 for the specific results.

[0236] S3: Thermogravimetric analysis of the solid material after drying the precursor slurry: starting temperatures are 80°C and 100°C, heating rate: 10°C / min, nitrogen protection, flow rate: 45mL / min.

[0237] The thermogravimetric curve obtained by the thermogravimetric analysis test is shown in FIG7 , wherein Y1 is the thermogravimetric (TG) curve and Y2 is the differential thermogravimetric (DTG) curve.

[0238] S4: spray-drying the precursor slurry at 100° C. to obtain precursor particles; then sintering the precursor particles in a nitrogen roller kiln at 750° C. to 790° C. for 20 hours to obtain lithium iron phosphate.

[0239] S5: X-ray diffraction test (XRD) and Fourier transform infrared test (FTIR) are performed on the prepared lithium iron phosphate, the solid material after drying the precursor slurry, and the ammonium salt compound respectively. Based on the characteristic peaks of the three, it can be determined whether ammonium salt compounds remain in the precursor slurry during the sintering heat treatment process and treatment.

[0240] Among them, the infrared curve comparison chart obtained by Fourier transform infrared testing (FTIR) of lithium iron phosphate, the solid material after drying the precursor slurry, and the ammonium salt compound is shown in Figure 8, wherein X1, X2, and X3 are the infrared curves of lithium iron phosphate, the solid material after drying the precursor slurry, and the ammonium salt compound, respectively.

[0241] Then, combined with the thermogravimetric curve information in step S3, it can be confirmed that the residual amount of ammonium salt compounds in the solid material after drying the precursor slurry is 34.7% at 400°C, 16.4% at 600°C, 1.71% and 0.34% at 700°C and 800°C, respectively.

[0242] Combined with the above test results, it can be seen that ammonium salt additives will remain in the precursor slurry during the sintering heat treatment process. After sintering is completed, the remaining amount is very small, and most of it is decomposed and vaporized.

[0243] S6: The prepared lithium iron phosphate was observed under a scanning electron microscope at different magnifications. The scanning electron micrographs are shown in Figure 9, where the scales for (a1) and (a2) are 1 μm and 20 μm, respectively. The images show well-formed spherical particles with smooth surfaces.

[0244] S7: The copper sulfate replacement method was used to test the elemental iron content in lithium iron phosphate. See Table 1 for the specific results.

[0245] S8: Preparation of secondary battery:

[0246] 1: Preparation of positive electrode

[0247] The lithium iron phosphate material, conductive agent, carbon nanotubes, and polyvinylidene fluoride prepared above were prepared in a mass ratio of 94:1.5:0.5:3 to prepare a positive electrode slurry, which was coated on a 13 μm thick Al foil. After vacuum drying at 120°C, cold pressing, and cutting into strips, a positive electrode sheet was obtained. The surface density of the positive electrode sheet was 2.4 mg / cm 2 .

[0248] 2: Preparation of electrolyte

[0249] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of EC:EMC:DEC = 1:1:1 to obtain an organic solvent, and then the fully dried lithium source LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0250] 3: Choose polyethylene film as the separator for lithium-ion batteries.

[0251] 4: Select a lithium sheet with a thickness of 12μm as the negative electrode sheet.

[0252] 5: Assembly of secondary batteries: stack the positive electrode sheet, separator, and negative electrode sheet in order, so that the separator is in the middle of the anode and cathode to play an isolating role, and wind them to obtain a bare cell. Place the bare cell in the outer packaging, inject the prepared electrolyte and perform the processes of packaging, liquid injection, formation, exhaust, etc. to obtain a secondary battery.

[0253] S9: Charging capacity test:

[0254] After the secondary battery was left to rest for 4 hours, it was placed on a blue battery tester for testing. The test process was as follows: constant current charging at 0.1C to 3.75V, constant voltage discharge at 3.75V until the current dropped to 50μA, and then rested for 5 minutes. Then, constant current discharge at 0.1C to 2V, and rested for 5 minutes.

[0255] 2) Perform step 1) twice, recording the charge capacity of the battery in each charging step or the discharge capacity in each discharging step during the test, calculating the cumulative charge capacity and the cumulative discharge capacity, dividing the cumulative charge capacity by the weight of lithium iron phosphate contained in the battery to obtain the charge capacity in grams, and dividing the cumulative discharge capacity by the weight of lithium iron phosphate contained in the battery to obtain the discharge capacity in grams.

[0256] Test tolerance: ±2mAh / g.

[0257] Please see Table 1 for specific results.

[0258] Examples 2-3

[0259] Examples 2 to 3 are substantially the same as Example 1, except that the type of ammonium salt compound used in step S1 is different from that in Example 1. Example 2 uses diammonium citrate, and Example 3 uses the ammonium salt compound shown in the following (b-1):

[0260] The preparation process of the ammonium salt compound shown in (b-1) is as follows:

[0261] Step 1: Dilute phosphoric acid with water to form dilute phosphoric acid, with a mass ratio of water to phosphoric acid of 1.3:1, and then add it to an enameled reaction tank with stirring and a jacket. Under stirring, introduce ammonia gas from a circular ammonia distributor for neutralization reaction. When the reaction liquid is neutralized to a pH of 8-9, filter it while hot to obtain diammonium hydrogen phosphate. Then, continuously add soluble citric acid to maintain the pH value of the reaction liquid at 5.0-7.4. Fully react for 12 hours to obtain a reaction product solution. Then, evaporation crystallization is controlled by heating in a crystallizer. The evaporation temperature is 55°C, and the supersaturation of the entire process is 1.02. The obtained crystals are centrifuged and finally dried at 55°C for 20 minutes to obtain a crystalline solid of the ammonium salt compound (b-1).

[0262] Among them, the structure of diammonium hydrogen phosphate is as follows:

[0263] The structure of citric acid is as follows:

[0264] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0265] Examples 4 to 9

[0266] Examples 4 to 9 are basically the same as Example 3, except that the amounts of the ammonium salt compound and deionized water in step S1 are regulated, and W1% is changed while keeping the solid content of the precursor slurry unchanged.

[0267] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0268] Among them, the lithium iron phosphate prepared in Example 5 was placed under a scanning electron microscope with different magnifications for observation, and the scanning electron microscope is shown in Figure 10, wherein the scales of the electron microscope images of (b1) and (b2) are 1 μm and 20 μm, respectively; the lithium iron phosphate prepared in Example 6 was placed under a scanning electron microscope with different magnifications for observation, and the scanning electron microscope is shown in Figure 11, wherein the scales of the electron microscope images of (c1) and (c2) are 1 μm and 20 μm, respectively; it can be seen from the figure that the spherical particles have perfect morphology and a smooth surface.

[0269] Examples 10 to 13

[0270] Examples 10 to 13 are substantially the same as Example 1, except that the amounts of glucose, ammonium salt compound, and deionized water in step S1 are regulated, and W1% and W3% are changed while keeping the solid content of the precursor slurry unchanged.

[0271] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0272] Example 14

[0273] Example 14 is basically the same as Example 1, with the only difference being that in step S1, the amount of deionized water in step S1 is regulated to change the solid content of the precursor slurry.

[0274] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0275] Examples 15-16

[0276] Examples 15 to 16 are substantially the same as Example 14, except that the amounts of the ammonium salt compound and deionized water in step S1 are adjusted to change W1% or W3% or the solid content. See Table 1 for details.

[0277] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0278] Comparative Examples 1-2

[0279] Comparative Examples 1 and 2 are substantially the same as Example 1, except that no ammonium salt compound is added in step S1, the amount of deionized water is regulated, and the solid content of the precursor slurry or W3% is changed.

[0280] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0281] The lithium iron phosphate prepared in Comparative Example 1 was observed under a scanning electron microscope at different magnifications. The scanning electron microscope images are shown in FIG12 , where the scales of the electron microscope images (d1) and (d2) are 1 μm and 20 μm, respectively, and the particle surface is relatively rough.

[0282] Comparative Example 3

[0283] Comparative Example 3 is substantially the same as Example 1, except that the type of ammonium salt compound used in step S1 is different from that in Example 1, and sodium citrate is used in Comparative Example 3.

[0284] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0285] The relevant parameters and performance test results of each embodiment and comparative example are shown in Table 1.

[0286] Table 1

[0287] Note: “ / ” means the substance does not exist.

[0288] By analyzing the data in Table 1 and comparing the data of the examples and comparative examples, it can be seen that the lithium iron phosphate particles prepared by the method for preparing lithium iron phosphate of the present application can improve the charge and discharge capacity of the secondary battery.

[0289] In the above-mentioned preparation method of lithium iron phosphate, the ammonium salt compound with a specific structure can play an excellent dispersing role. Even if the precursor slurry maintains a high solid content, lithium iron phosphate with good particle morphology and complete microstructure can be obtained.

[0290] Further analysis of the data from Examples 3 to 9 shows that, within a certain range, the greater the percentage W1% of the mass of the dispersing aid to the mass of the iron phosphate, the more complete the microstructure of the resulting lithium iron phosphate and the higher the charge and discharge capacity of the secondary battery. When W1% reaches approximately 1%, the growth rate of the charge and discharge capacity of the secondary battery essentially reaches equilibrium. Further increases in W1% result in a smaller growth rate of the charge and discharge capacity of the secondary battery, and when the charge and discharge capacity increases to a level that causes significant lithium deposition, this negatively impacts the life of the secondary battery. Therefore, the range of W1% can be further controlled to further increase the charge and discharge capacity of the secondary battery while reducing the probability of lithium deposition.

[0291] The above description of the various embodiments tends to emphasize the differences between the various embodiments. The same or similar aspects can be referenced with each other and will not be repeated herein for the sake of brevity.

[0292] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A method for preparing lithium iron phosphate, comprising the following steps: The iron phosphate, the lithium source, the carbon source, the dispersing aid and the solvent are mixed to prepare a precursor slurry; The precursor slurry is sintered to prepare lithium iron phosphate; in, The dispersing aid comprises an ammonium salt compound as shown in formula (1): R1 is a carbon-containing organic group.

2. The method for preparing lithium iron phosphate according to claim 1, wherein: The ammonium salt compound includes at least one of the compounds represented by formula (1A) to formula (1B): Among them, R 11 ~R 13 are independently selected from H, substituted or unsubstituted alkyl having 1 to 5 carbon atoms, hydroxyl or -C(O)O - NH4 + ; Each R2 and each R3 are independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms; L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH4 + Any of substituted alkane substituents having 1 to 5 carbon atoms.

3. The method for preparing lithium iron phosphate according to claim 2, wherein: In the compound represented by formula (1A), R 11 ~R 13 Each of the following is independently selected from H, an unsubstituted alkyl group having 1 to 5 carbon atoms, an alkyl group having 1 to 5 carbon atoms substituted with a hydroxyl group, a hydroxyl group, or -C(O)O - NH4 + .

4. The method for preparing lithium iron phosphate according to claim 2 or 3, wherein: In the compound represented by formula (1A), R 11 ~R 13 At least one of them is selected from -C(O)O - NH4 + .

5. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, wherein: The ammonium salt compound includes at least one of ammonium citrate, diamine citrate and compounds represented by formula (b-1) to (b-2): Among them, R 21 Selected from alkyl having 1 to 3 carbon atoms or -C(O)O - NH4 + .

6. The method for preparing lithium iron phosphate according to any one of claims 1 to 5, wherein: The mass of the dispersing aid accounts for 0.3% to 7% of the mass of the ferric phosphate.

7. The method for preparing lithium iron phosphate according to claim 6, wherein: The mass of the dispersing aid accounts for 0.5% to 1% of the mass of the ferric phosphate.

8. The method for preparing lithium iron phosphate according to any one of claims 1 to 7, wherein: The sintering temperature is 780°C to 820°C.

9. The method for preparing lithium iron phosphate according to claim 8, wherein: The sintering temperature is 750°C to 790°C.

10. The method for preparing lithium iron phosphate according to any one of claims 1 to 9, wherein: The carbon source comprises at least one of glucose, sucrose, starch, phenolic resin, cyclodextrin, polyethylene, polyethylene glycol and polyvinyl alcohol.

11. The method for preparing lithium iron phosphate according to any one of claims 1 to 10, wherein: The lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

12. The method for preparing lithium iron phosphate according to any one of claims 1 to 11, wherein: The solid content of the precursor slurry is ≥45%.

13. The method for preparing lithium iron phosphate according to any one of claims 1 to 12, wherein: The solvent includes at least one of water and an alcohol organic solvent.

14. A lithium iron phosphate, wherein the lithium iron phosphate is prepared by the method for preparing lithium iron phosphate according to any one of claims 1 to 13.

15. An ammonium salt compound, wherein the ammonium salt compound is represented by formula (1B): Each R2 and each R3 are independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms; L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH4 + Any of substituted alkane substituents having 1 to 5 carbon atoms.

16. Use of the ammonium salt compound according to claim 15, wherein the ammonium salt compound is used as a dispersing aid.

17. Use of the ammonium salt compound according to claim 15 in the preparation of lithium iron phosphate.

18. A positive electrode sheet, comprising a current collector and a positive electrode active layer disposed on a surface of the current collector, wherein a component of the positive electrode active layer comprises the lithium iron phosphate as claimed in claim 14. 19 . A secondary battery, comprising the lithium iron phosphate according to claim 14 or the positive electrode sheet according to claim 18 .

20. An electric device, comprising the secondary battery according to claim 19.

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