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

Abstract

The invention relates to lithium iron phosphate, a preparation method and application thereof, an ammonium salt compound and application thereof. The preparation method of 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 precursor slurry; carrying out sintering treatment on the precursor slurry to prepare lithium iron phosphate; wherein the dispersing aid comprises an ammonium salt compound as shown in a formula (1), wherein R1 is a carbon-containing organic group. The lithium iron phosphate prepared by the preparation method of the lithium iron phosphate can improve the charge-discharge capacity of a secondary battery.

Description

Technical Field

[0001] The present invention relates to the field of battery technology, and in particular to lithium iron phosphate and a preparation method and application thereof, and an ammonium salt compound and application thereof. Background Art

[0002] 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 electrical energy.

[0003] Lithium iron phosphate cathode material has become one of the most widely used cathode materials due to its stable structure, excellent charge and discharge platform, high capacity, and good cycle performance. The traditional preparation of lithium iron phosphate is mainly synthesized by 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 leads to an increase in the viscosity of the slurry, which is not conducive to the reaction to generate lithium iron phosphate, making the microstructure of the prepared lithium iron phosphate incomplete. The low solid content of the slurry greatly limits the production capacity of lithium iron phosphate, and the traditional preparation process is accompanied by too many by-product reactions. These factors will reduce the stability of lithium iron phosphate. When used in the preparation of batteries, it will have a negative impact on the performance of the battery, such as charge and discharge capacity.

[0004] As people's demand for secondary batteries increases, traditional lithium iron phosphate positive electrode materials are increasingly unable to meet people's needs and need further improvement. Summary of the invention

[0005] Based on this, it is necessary to provide a lithium iron phosphate and its preparation method and application, ammonium salt compound and its application, which are beneficial to improve the charge and discharge capacity of secondary batteries.

[0006] In a first aspect of the present application, a method for preparing lithium iron phosphate is provided, comprising the following steps:

[0007] The iron phosphate, the lithium source, the carbon source, the dispersing aid and the solvent are mixed to prepare a precursor slurry;

[0008] The precursor slurry is sintered to prepare lithium iron phosphate;

[0009] Wherein, the dispersing aid comprises an ammonium salt compound as shown in formula (1):

[0010]

[0011] R 1 A carbon-containing organic group.

[0012] 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 of a specific structure. The ammonium salt compound will form a negatively charged ion group after ionization. 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 probability of agglomeration between solid particles in the precursor slurry is reduced through the steric hindrance, which is beneficial to the reaction of components in the subsequent sintering treatment, and improves the microstructural integrity of the obtained lithium iron phosphate. 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.

[0013] 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.

[0014] In some embodiments, the ammonium salt compound includes at least one of the compounds represented by formula (1A) to formula (1B):

[0015]

[0016] 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 - NH 4 + ;

[0017] Each R 2 and each R 3 Each of them is independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0018] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH 4 + Any of substituted alkane substituents having 1 to 5 carbon atoms.

[0019] 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 with relatively high activity. 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.

[0020] Furthermore, since the compound of formula (1B) promotes the reaction of 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 can further improve the charge and discharge capacity of the secondary battery.

[0021] In some embodiments, the compound represented by formula (1A) satisfies at least one of the following conditions (1) to (2):

[0022] (1)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 - NH 4 + ;

[0023] (2)R 11 ~R 13 At least one of them is selected from -C(O)O - NH 4 + .

[0024] In the compound represented by formula (1A), -C(O)O - NH 4 + The more groups there are, the stronger its dispersing ability is.

[0025] 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):

[0026]

[0027] Among them, R 21 Selected from alkyl having 1 to 3 carbon atoms or -C(O)O - NH 4 + .

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

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

[0030] By adjusting 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 of the mass of the dispersing aid to the mass of the iron phosphate, the more complete the microstructure of the lithium iron phosphate obtained, and the higher the charge and discharge capacity of the secondary battery. When W1% reaches about 1%, the growth rate of the charge and discharge capacity of the secondary battery basically reaches a balance. If W1% continues to increase, the growth rate of the charge and discharge capacity of the secondary battery is small, and the charge and discharge capacity increases to a level that causes more lithium precipitation, which will have a negative impact on 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 precipitation.

[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 alcohol organic solvent.

[0040] In a second aspect of the present application, a lithium iron phosphate is provided. The lithium iron phosphate is prepared by the preparation method of the lithium iron phosphate of 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 of a specific structure. The ammonium salt compound will form a negatively charged ion group after ionization. The negatively charged end is adsorbed on the surface of the component iron phosphate in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches out 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]

[0044] Each R 2 and each R 3 Each of them is independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0045] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH 4 + Any of substituted alkane substituents having 1 to 5 carbon atoms.

[0046] 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.

[0047] 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.

[0048] The compound of formula (1B) also contains a specific phosphate group structure with relatively high activity. When used in the preparation of lithium iron phosphate, it can not only play a dispersing role, but also promote the reaction of iron phosphate, lithium source and carbon source, promote the formation of lithium iron phosphate, and further improve the microstructural stability of the prepared lithium iron phosphate.

[0049] Furthermore, since the compound of formula (1B) promotes the reaction of 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 can further improve the charge and discharge capacity of the secondary battery.

[0050] In a sixth aspect of the present application, a positive electrode sheet is provided, the 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 of the second aspect.

[0051] 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.

[0052] According to an eighth aspect of the present application, there is provided an electrical device, wherein the electrical device comprises the secondary battery according to the seventh aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Moreover, the same reference numerals are used throughout the drawings to represent the same components. In the drawings:

[0054] Figure 1 is a schematic diagram of the mechanism of action of a dispersing aid in the preparation process of lithium iron phosphate in one embodiment;

[0055] Figure 2 is a schematic diagram of one embodiment of a battery cell;

[0056] Figure 3 yes Figure 2 Exploded diagram of

[0057] Figure 4 is a schematic diagram of an embodiment of a battery pack;

[0058] Figure 5 yes Figure 4 Exploded diagram of

[0059] Figure 6 is a schematic diagram of an embodiment of an electric device using a secondary battery as a power source;

[0060] Figure 7 This is a thermogravimetric curve of the solid material after the precursor slurry in Example 1 is dried;

[0061] Figure 8 This is a comparison chart of infrared curves obtained by Fourier transform infrared testing (FTIR) of the lithium iron phosphate prepared in Example 1, the solid material after the precursor slurry is dried, and the ammonium salt compound;

[0062] Fig. 9 This is a scanning electron microscope image of the lithium iron phosphate prepared in Example 1;

[0063] Fig.10 This is a scanning electron microscope image of the lithium iron phosphate prepared in Example 5;

[0064] Fig.11 This is a scanning electron microscope image of the lithium iron phosphate prepared in Example 6;

[0065] Fig.12 This is a scanning electron microscope image of the lithium iron phosphate prepared in Comparative Example 1.

[0066] Description of reference numerals:

[0067] 1. Battery pack; 2. Upper box; 3. Lower box; 4. Battery cell; 41. Shell; 42. Electrode assembly; 43. Cover plate; 5. Electrical device. DETAILED DESCRIPTION

[0068] In order to make the above-mentioned purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are described in detail below. In the following description, many specific details are set forth to facilitate a full understanding of the present application. However, the present application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar improvements without violating the connotation of the present application, so the present application is not limited by the specific embodiments disclosed below.

[0069] In addition, the terms "first" and "second" are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" and "second" may explicitly or implicitly include at least one of the features. In the description of this application, the meaning of "plurality" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0070] In the present 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.

[0071] In the present application, the carbon number of "alkyl group having 1 to 5 carbon atoms" may be 1 to 5, including 1, 2, 3, 4, 5, and non-limiting examples include methane, ethane, and n-propane.

[0072] 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.

[0073] In traditional technology, the preparation of traditional lithium iron phosphate is mainly synthesized by phosphorus iron process, and the main process is: iron phosphate, lithium source, and carbon source are mixed in a solvent to prepare slurry, and then sintered. In order to increase the solid content of the slurry to increase production capacity, dispersants are often added during the mixing process to reduce the probability of slurry agglomeration.

[0074] 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 needs to be heated to flow, which makes them unsuitable for industrial production.

[0075] Other technologies use alkali metal salts of small molecular 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.

[0076] Thus, after a lot of exploration, the lithium iron phosphate and its preparation method in this application were obtained.

[0077] In one embodiment of the present application, a method for preparing lithium iron phosphate is provided, comprising the following steps S10 to S20.

[0078] Step S10: Mixing iron phosphate, a lithium source, a carbon source, a dispersing aid and a solvent to prepare a precursor slurry.

[0079] Step S20: sintering the precursor slurry to prepare lithium iron phosphate.

[0080] Wherein, the dispersing aid includes an ammonium salt compound as shown in formula (1):

[0081]

[0082] R 1 A carbon-containing organic group.

[0083] 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 of a specific structure. The ammonium salt compound will form a negatively charged ion group after ionization. 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 is stretched to form a steric hindrance. The probability of agglomeration between solid particles in the precursor slurry is reduced through the steric hindrance, 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.

[0084] In the preparation method of the 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.

[0085] Please refer to Figure 1 , Figure 1 Schematic diagram of the mechanism of action 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 component iron phosphate in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches to form a steric hindrance.

[0086] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-5000.

[0087] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-1000.

[0088] In some embodiments, the relative molecular mass of the ammonium salt compound is 100-500.

[0089] "Relative molecular mass" refers to the sum of the relative atomic masses (Ar) of the individual atoms in a chemical formula.

[0090] In some embodiments, the ammonium salt compound comprises at least one of the compounds represented by formula (1A) to formula (1B):

[0091]

[0092] 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 - NH 4 + .

[0093] Each R 2 and each R 3 Each of them is independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms;

[0094] L is selected from a single bond, an alkane subunit having 1 to 5 carbon atoms, -C(O)O - NH 4 + Any of substituted alkane substituents having 1 to 5 carbon atoms.

[0095] 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 with relatively high activity. 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.

[0096] Furthermore, since the compound of formula (1B) promotes the reaction of 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 can further improve the charge and discharge capacity of the secondary battery.

[0097] In some embodiments, 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 - NH 4 + .

[0098] In some embodiments, R 11 ~R 13 Each of the following is independently selected from H, an unsubstituted alkyl radical having 1 to 5 carbon atoms, an alkyl radical having 1 to 5 carbon atoms substituted with a hydroxyl radical, a hydroxyl radical, or -C(O)O - NH 4 + .

[0099] In some embodiments, R 11 ~R 13 Each of the following is independently selected from H, an unsubstituted alkyl radical having 1 to 3 carbon atoms, an alkyl radical having 1 to 3 carbon atoms substituted with a hydroxyl radical, a hydroxyl radical, or -C(O)O - NH 4 + .

[0100] In some embodiments, R 11 ~R 13 are independently selected from H, methyl, ethyl, propyl, hydroxyl or -C(O)O - NH 4 + .

[0101] In some embodiments, R 11 ~R 13 At least one of them is selected from -C(O)O - NH 4 + .

[0102] In the compound represented by formula (1A), -C(O)O - NH 4 + The more groups there are, the stronger its dispersing ability is.

[0103] In some embodiments, R 11 ~R 13 At least two of them are selected from -C(O)O - NH 4 + .

[0104] In some embodiments, each R 2 and each R3 Each is independently selected from any one of H and an unsubstituted alkyl group having 1 to 5 carbon atoms.

[0105] In some embodiments, each R 2 and each R 3 Each of them is independently selected from any one of H and an unsubstituted alkyl group having 1 to 5 carbon atoms.

[0106] In some embodiments, each R 2 and each R 3 Each is independently selected from any one of H and an unsubstituted alkyl group having 1 to 3 carbon atoms.

[0107] In some embodiments, each R 2 same.

[0108] In some embodiments, each R 3 same.

[0109] In some embodiments, R 2 and R 3 same.

[0110] In some embodiments, L is selected from a single bond or -C(O)O - NH 4 + The substituted alkane subunit has 1 to 5 carbon atoms.

[0111] In some embodiments, L is selected from a single bond or -C(O)O - NH 4 + The substituted alkane subunit has 1 to 3 carbon atoms.

[0112] The preparation method of the compound of formula (1B) is as follows:

[0113] Compound (a) and diammonium hydrogen phosphate are mixed and reacted to prepare a compound of formula (1B).

[0114] The structure of diammonium hydrogen phosphate is as follows:

[0115]

[0116] The structure of compound (a) is as follows:

[0117]

[0118] In some embodiments, during the mixing reaction, the pH value of the reaction solution is maintained at 5.0-7.4.

[0119] In some embodiments, the mixing reaction time is 4 hours to 24 hours.

[0120] Condensation and dehydration reactions occur during the mixing reaction, including condensation esterification and condensation to etherification reactions.

[0121] L, each R 2 and each R 3 The type of selection is the same as above and will not be repeated here.

[0122] In some embodiments, the ammonium salt compound includes at least one of the compounds represented by formula (2A) to formula (2B):

[0123]

[0124] 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):

[0125]

[0126] Among them, R 21 Selected from alkyl having 1 to 3 carbon atoms or -C(O)O - NH 4 + .

[0127] In some embodiments, R 21 Selected from -C(O)O - NH 4 + .

[0128] In some embodiments, the mass of the dispersing aid accounts for 0.3% to 7% of the mass of the ferric phosphate.

[0129] In some embodiments, the mass of the dispersing aid accounts for 0.5% to 1% of the mass of the ferric phosphate.

[0130] By adjusting the dosage of the dispersing agent, the microstructural stability of the prepared lithium iron phosphate is further improved, and the sphericity is high.

[0131] 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%, 5.5%, 5.6%, 5.7%, 5.8%, 5.9%, 6.1%, 6.2%, 6.3%, 6.4%, 6.5%, 6.6%, 6.7%, 6.8%, 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%.

[0132] In some embodiments, the sintering temperature is 780°C to 820°C.

[0133] Optionally, the sintering temperature is 750°C to 790°C.

[0134] 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 can further improve the charge and discharge capacity of the secondary battery.

[0135] In the above "780℃~820℃", the values ​​include the minimum and maximum values ​​of the range, and 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.

[0136] In some of the embodiments, the sintering time is 19 h to 21 h.

[0137] In some embodiments, in the mixing process of step S10, the specific steps are as follows:

[0138] mixing a dispersing aid and a solvent to form a mixed mixture;

[0139] The iron phosphate, the lithium source, the carbon source and the mixed mixture are mixed to prepare a slurry.

[0140] In some of the embodiments, the mixing process is carried out under stirring conditions for a time of 0.5 h to 1 h.

[0141] In some of the embodiments, before the sintering step, the method further includes spray drying the precursor slurry.

[0142] After spray drying, a solid precursor in the form of spherical particles can be obtained.

[0143] In some embodiments, the spray drying temperature is 280°C to 350°C.

[0144] In some of the 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 ).

[0145] In some of the embodiments, the mass of the lithium source accounts for 0.235% to 0.25% of the mass of the iron phosphate.

[0146] In some of the embodiments, the mass of the lithium source accounts for 0.237% to 0.25% of the mass of the iron phosphate.

[0147] In some embodiments, the mass of the carbon source accounts for 5% to 15% of the mass of the ferric phosphate.

[0148] In some embodiments, the mass of the carbon source accounts for 5% to 10% of the mass of the ferric phosphate.

[0149] In some of the embodiments, the mass of the carbon source accounts for 5% to 9% of the mass of the ferric phosphate.

[0150] In some of the embodiments, the mass of the carbon source accounts for 6% to 9% of the mass of the ferric phosphate.

[0151] In some embodiments, the carbon source includes at least one of glucose, sucrose, starch, phenolic resin, cyclodextrin, polyethylene, polyethylene glycol and polyvinyl alcohol.

[0152] In some embodiments, the lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

[0153] In some embodiments, the solid content of the precursor slurry is greater than or equal to 45%.

[0154] In some embodiments, the solid content of the precursor slurry is 45% to 55%.

[0155] In the preparation method of the above-mentioned 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 the production capacity.

[0156] In some embodiments, the solvent includes at least one of water and an alcohol organic solvent.

[0157] In some embodiments, the solvent includes water.

[0158] In one embodiment of the present application, a lithium iron phosphate is further provided. The lithium iron phosphate is prepared by the above-mentioned preparation method of lithium iron phosphate.

[0159] In the preparation method of the above-mentioned lithium iron phosphate preparation method, the dispersing aid includes an ammonium salt compound of a specific structure. The ammonium salt compound will form a negatively charged ion group after ionization. The negatively charged end is adsorbed on the surface of the component iron phosphate in the precursor slurry, and the carbon-containing organic group at the other end of the group stretches out 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.

[0160] In one embodiment of the present application, an ammonium salt compound is provided, and the ammonium salt compound is as shown in formula (1B):

[0161]

[0162] Each R 2 and each R 3 Each is independently selected from any one of H and a substituted or unsubstituted alkyl group having 1 to 5 carbon atoms.

[0163] The above R 2 and each R 3 The type selection is the same as described above and will not be repeated here.

[0164] The preparation method of the ammonium salt compound represented by formula (1B)) is the same as described above and will not be repeated here.

[0165] The present application also provides the use of the above ammonium salt compound in the preparation of lithium iron phosphate.

[0166] The compound of formula (1B) also contains a specific phosphate group structure with relatively high activity. When used in the preparation of lithium iron phosphate, it can not only play a dispersing role, but also promote the reaction of iron phosphate, lithium source and carbon source, promote the formation of lithium iron phosphate, and further improve the microstructural stability of the prepared lithium iron phosphate.

[0167] Furthermore, since the compound of formula (1B) promotes the reaction of 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 can further improve the charge and discharge capacity of the secondary battery.

[0168] In some of the embodiments, the above ammonium salt compound is used as a dispersing aid.

[0169] In one embodiment of the present application, a positive electrode sheet is further provided. The positive electrode sheet includes a current collector and a positive electrode active layer disposed on a surface of the current collector. The component of the positive electrode active layer includes the above-mentioned lithium iron phosphate.

[0170] In some of the 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 %.

[0171] 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.

[0172] The positive electrode conductive agent can be a conductive agent 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 be selected from SP, KS-6, acetylene black, branched Ketjen black ECP, SFG-6, vapor-grown carbon fiber VGCF, carbon nanotubes CNTs and graphene and their composite conductive agents.

[0173] 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.

[0174] 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), carboxymethyl chitosan (CMCS) and at least one of fluorine-containing acrylate resins.

[0175] 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.

[0176] In any embodiment of the present application, the positive electrode sheet can be prepared by the following method: the components for preparing the positive electrode sheet are dispersed in a solvent (such as N-methylpyrrolidone) to form a positive electrode slurry; the positive electrode slurry is coated on the current collector, and after drying, cold pressing and other processes, the positive electrode sheet can be obtained. The solid content of the positive electrode slurry is 40wt% to 80wt%, and the viscosity at room temperature is adjusted to 5000mPa·s

[0177] ~25000mPa·s, the positive electrode slurry is coated on the surface of the positive electrode collector, and after drying, it is cold-pressed by a cold rolling mill to form a positive electrode sheet.

[0178] In some embodiments, the compaction 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:

[0179] Compacted density = coating surface density / (thickness of the electrode after extrusion - thickness of the current collector).

[0180] 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.

[0181] The secondary battery further includes a negative electrode sheet, a separator and an electrolyte. Examples of the negative electrode sheet, the separator and the electrolyte are described below, including but not limited to the following.

[0182] Electrolyte: Generally, the electrolyte includes electrolyte salt and solvent.

[0183] In some embodiments, the electrolyte salt may be selected from electrolyte salts commonly used in the art, such as lithium ion electrolyte salts.

[0184] As examples, lithium ion electrolyte salts include, but are not limited to, lithium hexafluorophosphate (LiPF 6 ), lithium tetrafluoroborate (LiBF 4 ), lithium perchlorate (LiClO 4 ) lithium hexafluoroarsenate (LiAsF 6 ), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO 2 F 2 ), one or more of lithium difluorooxalate phosphate (LiDFOP) and lithium tetrafluorooxalate phosphate (LiTFOP).

[0185] 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), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0186] In some embodiments, in the electrolyte solution, the concentration of the electrolyte salt is generally 0.5 mol / L to 15 mol / L.

[0187] In some embodiments, the electrolyte may further include additives, such as 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, additives that improve battery high or low temperature performance, etc.

[0188] Separator: The separator is placed between the positive electrode and the negative electrode.

[0189] 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.

[0190] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different.

[0191] The thickness of the diaphragm is controlled within a range of 2 μm to 15 μm; optionally, the thickness of the diaphragm is controlled within a range of 2 μm to 13 μm.

[0192] Negative electrode sheet: The negative electrode sheet can be a negative electrode sheet for various secondary battery systems in the art.

[0193] In some of the embodiments, the secondary battery is a lithium metal secondary battery, and the negative electrode sheet can be a negative electrode sheet known in the art that can be used for lithium metal batteries.

[0194] In some embodiments, the negative electrode sheet directly adopts a lithium-containing metal sheet.

[0195] In another embodiment, the negative electrode sheet includes a lithium-containing metal layer and a conductive layer stacked together.

[0196] Furthermore, the lithium-containing metal in the lithium-containing metal sheet and the lithium-containing metal layer may be lithium metal, or an alloy formed by lithium metal and other metal or non-metal elements.

[0197] 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).

[0198] In some of these embodiments, the conductive layer may be a copper foil.

[0199] In any embodiment of the present application, the negative electrode sheet can be prepared in the following manner: directly pressing a lithium-containing metal sheet to obtain the negative electrode sheet, or stacking and pressing the lithium-containing metal layer and the conductive layer to obtain the negative electrode sheet.

[0200] In some of the embodiments, the secondary battery is a lithium-ion secondary battery, and the negative electrode sheet can be a negative electrode sheet known in the art that can be used for a lithium-ion battery.

[0201] 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.

[0202] The components of the negative electrode active layer include a negative electrode active material.

[0203] The negative electrode active material mentioned above can adopt the commonly used negative electrode active materials in this application.

[0204] In any embodiment of the present application, the above-mentioned negative electrode active material includes at least one of mesophase carbon 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.

[0205] Optionally, specific examples of the above-mentioned negative electrode active materials include, but are not limited to: at least one of mesophase carbon 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.

[0206] In any embodiment of the present application, the mass proportion of the above-mentioned negative electrode active material in the negative electrode active layer is 70% to 100%.

[0207] 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.

[0208] 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, superconducting carbon black Ketjen black (ECP) with a branched structure, vapor-grown carbon fiber (VGCF), carbon nanotubes (CNTs) and graphene and composite conductive agents thereof.

[0209] 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.

[0210] The above-mentioned negative electrode binder can be a commonly used binder 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).

[0211] 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.

[0212] In any embodiment of the present application, the negative electrode active layer may further include other additives, such as thickeners, such as sodium carboxymethyl cellulose (CMC-Na), etc. Based on the total weight of the negative electrode active layer, the weight ratio of other additives in the negative electrode active layer is 0-15wt%.

[0213] 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.

[0214] The composite current collector may include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector may be formed by forming a metal material on a polymer material substrate.

[0215] 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.

[0216] In some of the embodiments, the polymer material substrate includes at least one of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0217] In any embodiment of the present application, the negative electrode sheet can be prepared in the following manner: 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 collector, and after drying, cold pressing and other processes, the negative electrode sheet can be obtained.

[0218] In some embodiments, the solvent includes but is not limited to water.

[0219] In some of the 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.

[0220] 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 .

[0221] The surface density of the negative electrode active material = the mass of the negative electrode active material / the area of ​​the negative electrode sheet.

[0222] The secondary battery of the present application may be in a cylindrical, square or other arbitrary shapes. For example, Figure 2 The battery cell 4 is a square structure as an example.

[0223] In some embodiments, reference Figure 3 The battery cell 4 includes a shell and an electrode assembly 42, and the shell may include a shell 41 and a cover plate 43. The shell 41 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 41 has an opening connected to the receiving cavity, and the cover plate 43 can be covered on the opening to close the receiving cavity.

[0224] The positive electrode sheet, the negative electrode sheet and the separator can be wound or laminated to form an electrode assembly 42. The electrode assembly 42 is encapsulated in the receiving cavity. The electrolyte is infiltrated in the electrode assembly 42. The number of electrode assemblies 42 contained in the battery cell 4 can be one or more, which can be adjusted according to needs.

[0225] The present application also provides an electrical device, which includes the secondary battery mentioned above.

[0226] 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.

[0227] Figure 3 and Figure 4The battery pack 1 is used as an example. The battery pack 1 includes a battery box and one or more battery cells 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3. The upper box body 2 can cover the lower box body 3 and form a closed space for the battery cells 4.

[0228] The plurality of battery cells 4 may be arranged in the battery box in any manner.

[0229] 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.

[0230] The above-mentioned electrical devices may be, but are not limited to, mobile equipment, electric vehicles, electric trains, ships and satellites, energy storage systems, etc.

[0231] In some of these embodiments, the mobile device may be a mobile phone or a laptop computer, etc.

[0232] In some of the embodiments, the electric vehicle includes, but is not limited to, a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.

[0233] Figure 5 The power consumption device 5 is taken as an example. The power consumption device 5 is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the power consumption device 5's requirements for high power and high energy density of the battery, a battery pack can be used.

[0234] As another example, the power-consuming device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a battery may be used as a power source.

[0235] 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, technical personnel in the field 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.

[0236] The following are specific embodiments.

[0237] Example 1

[0238] S1: Preparation of lithium iron phosphate

[0239] (1) An ammonium salt compound (ammonium citrate) is dissolved in deionized water to obtain a mixture.

[0240] (2) Put iron phosphate, lithium carbonate and glucose into a stirring tank, add the mixture into the stirring tank while stirring with a paddle, stir and grind for 20 minutes to mix evenly, and obtain a precursor slurry. The mass of the ammonium salt compound in the mixture is W1% of the mass of the iron phosphate, the mass of the lithium carbonate is W2% of the mass of the iron phosphate, and the mass of the glucose is W3% of the mass of the iron phosphate. See Table 1 for details.

[0241] S2: Solid content test

[0242] (1) Sample weighing: Weigh a certain amount of sample (10 g), record the accuracy and resolution of the weighing device, and record the sample weight as M0.

[0243] (2) Drying: Adjust the set temperature to 100°C and place the sample in an oven for drying until the mass no longer changes. Record the height of the load bar within a fixed time and record the weight after drying as M1.

[0244] (3) Cooling: Cool the measuring cup to room temperature, remove condensed water and then weigh it.

[0245] (4) Calculate the solid content: Solid content = M1 / M0 × 100%. Please see Table 1 for the specific results.

[0246] S3: Thermogravimetric analysis test was performed on the solid material after the precursor slurry was dried: the starting temperatures were 80°C and 100°C, respectively, the heating rate was 10°C / min, nitrogen protection, and the flow rate was 45mL / min.

[0247] Thermogravimetric curves obtained by thermogravimetric analysis are shown in Figure 2. Figure 7 As shown, Y1 is the thermogravimetric (TG) curve and Y2 is the thermogravimetric differential (DTG) curve.

[0248] S4: spray drying the above 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.

[0249] 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.

[0250] Among them, the infrared curve comparison diagram of lithium iron phosphate, the solid material after drying the precursor slurry and the ammonium salt compound obtained by Fourier transform infrared test (FTIR) is as follows Figure 8 As shown, X1, X2 and X3 are infrared curves of lithium iron phosphate, solid material after drying of precursor slurry and ammonium salt compound respectively.

[0251] 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 the precursor slurry is dried is 34.7% at 400°C, 16.4% at 600°C, and 1.71% and 0.34% at 700°C and 800°C, respectively.

[0252] 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, the remaining amount is very small, and most of it is decomposed and vaporized.

[0253] S6: The prepared lithium iron phosphate is placed under a scanning electron microscope with different magnifications for observation. Fig. 9 As shown, the scales of the electron microscope images (a1) and (a2) are 1 μm and 20 μm respectively. From the figure, it can be seen that the spherical particles have perfect morphology and smooth surface.

[0254] S7: The copper sulfate replacement method was used to test the elemental iron content in lithium iron phosphate. See Table 1 for specific results.

[0255] S8: Preparation of secondary battery:

[0256] 1: Preparation of positive electrode

[0257] The lithium iron phosphate material, conductive agent, carbon nanotubes and polyvinylidene fluoride prepared above were prepared into positive electrode slurry according to the formula of mass ratio of 94:1.5:0.5:3, coated on 13 μm thick Al foil, vacuum dried at 120°C, cold pressed and cut into strips to obtain positive electrode sheets, wherein the surface density of the positive electrode sheet was 2.4 mg / cm 2 .

[0258] 2: Preparation of electrolyte

[0259] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of EC:EMC:DEC=1:1:1 to obtain an organic solvent. 6 Dissolve in the above organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0260] 3: Choose polyethylene film as the isolation membrane for lithium-ion batteries.

[0261] 4: Select a lithium sheet with a thickness of 12μm as the negative electrode sheet.

[0262] 5: Assembly of secondary batteries: stack the positive electrode sheet, isolation film, and negative electrode sheet in order, so that the isolation film is between the positive and negative electrodes to play a role of isolation, and wind them to get a bare cell. Place the bare cell in an outer package, inject the prepared electrolyte, and perform packaging, liquid injection, formation, exhaust and other processes to obtain a secondary battery.

[0263] S9: Charging gram capacity test:

[0264] After the secondary battery was left to stand 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 50uA, and then left to stand for 5 minutes, and then constant current discharge at 0.1C until the voltage was 2V, and then left to stand for 5 minutes;

[0265] 2) Perform step 1) twice, record the charging capacity of the battery in each charging step or the discharging capacity in each discharging step during the test, calculate the cumulative charging capacity and the cumulative discharging capacity, divide the cumulative charging capacity by the weight of lithium iron phosphate contained in the battery to obtain the charging capacity in grams, and divide the cumulative discharging capacity by the weight of lithium iron phosphate contained in the battery to obtain the discharging capacity in grams.

[0266] Test tolerance: ±2mAh / g.

[0267] Please see Table 1 for specific results.

[0268] Embodiment 2-3

[0269] Embodiments 2 to 3 are basically the same as Embodiment 1, except that the type of ammonium salt compound used in step S1 is different from that in Embodiment 1. Embodiment 2 uses diammonium citrate, and Embodiment 3 uses the ammonium salt compound shown in (b-1) below:

[0270]

[0271] The preparation process of the ammonium salt compound shown in (b-1) is as follows:

[0272] Step 1: Dilute phosphoric acid with water to make dilute phosphoric acid, the mass ratio of water to phosphoric acid is 1.3:1, then add it to an enameled reaction tank with stirring and jacket, and introduce ammonia through a circular tube ammonia distributor for neutralization reaction under stirring. When the pH of the reaction liquid is neutralized to 8-9, filter it while hot to obtain diammonium hydrogen phosphate, and then continuously drip soluble citric acid to keep 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 whole process is 1.02. The obtained crystals are centrifuged and finally dried at 55°C for 20 minutes to obtain a crystalline solid of (b-1) ammonium salt compound.

[0273] Among them, the structure of diammonium hydrogen phosphate is as follows:

[0274]

[0275] The structure of citric acid is as follows:

[0276]

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

[0278] Embodiments 4 to 9

[0279] Examples 4 to 9 are basically the same as Example 3, except that the amount of the ammonium salt compound and deionized water in step S1 is adjusted to change W1% while keeping the solid content of the precursor slurry unchanged.

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

[0282] Embodiments 10 to 13

[0283] 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 the solid content of the precursor slurry remains unchanged.

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

[0285] Embodiment 14

[0286] Example 14 is basically the same as Example 1, except that in step S1, the amount of deionized water in step S1 is regulated to change the solid content of the precursor slurry.

[0287] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0288] Embodiments 15-16

[0289] Examples 15 to 16 are basically the same as Example 14, except that the amount of the ammonium salt compound and deionized water in step S1 is adjusted to change W1% or W3% or the solid content. See Table 1 for details.

[0290] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0291] Comparative Examples 1-2

[0292] Comparative Examples 1 to 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 adjusted, and the solid content or W3% of the precursor slurry is changed.

[0293] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0294] The lithium iron phosphate prepared in Comparative Example 1 was observed under scanning electron microscopes at different magnifications. Fig.12 As shown, the scales of the electron microscope images (d1) and (d2) are 1 μm and 20 μm, respectively, and the particle surface is relatively rough.

[0295] Comparative Example 3

[0296] 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.

[0297] The other steps are the same as those in Example 1. Please see Table 1 for specific parameters.

[0298] The relevant parameters and performance test results of each embodiment and comparative example are shown in Table 1.

[0299] Table 1

[0300]

[0301]

[0302] Note: “ / ” means the substance does not exist.

[0303] By analyzing the data in Table 1 and comparing the data of the embodiments 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.

[0304] Furthermore, 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 maintains a high solid content, lithium iron phosphate with good particle morphology and complete microstructure can be obtained.

[0305] Furthermore, further analysis of the data of Examples 3 to 9 shows that within a certain range, the greater the percentage of the mass of the dispersing aid to the mass of the iron phosphate, the more complete the microstructure of the lithium iron phosphate obtained, and the higher the charge and discharge capacity of the secondary battery. When W1% reaches about 1%, the growth rate of the charge and discharge capacity of the secondary battery basically reaches a balance. If W1% continues to increase, the growth rate of the charge and discharge capacity of the secondary battery is small, and the charge and discharge capacity increases to a level that causes more lithium precipitation, which will have a negative impact on 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 precipitation.

[0306] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0307] The above-mentioned embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application shall be subject to the attached claims, and the description and drawings may be used to interpret the contents of the claims.

Claims

1. A method for preparing lithium iron phosphate, characterized in that: The steps include: 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; Wherein, 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, characterized in that: 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, characterized in that: The compound represented by formula (1A) satisfies at least one of the following conditions (1) to (2): (1)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 + ; (2)R 11 ~R 13 At least one of them is selected from -C(O)O - NH4 + .

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

5. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The mass of the dispersing aid accounts for 0.3% to 7% of the mass of the iron phosphate; Optionally, the mass of the dispersing aid accounts for 0.5% to 1% of the mass of the ferric phosphate.

6. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The sintering temperature is 780°C to 820°C; Optionally, the sintering temperature is 750°C to 790°C.

7. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The preparation method satisfies at least one of the following conditions (1) to (2): (1) The carbon source comprises at least one of glucose, sucrose, starch, phenolic resin, cyclodextrin, polyethylene, polyethylene glycol and polyvinyl alcohol; (2) The lithium source includes at least one of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium phosphate and lithium acetate.

8. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The precursor slurry satisfies at least one of the following conditions (1) to (3): (1) The solid content of the precursor slurry is ≥45%; (2) The solvent includes at least one of water and an alcohol organic solvent.

9. A lithium iron phosphate, characterized in that: The lithium iron phosphate is prepared by the method for preparing lithium iron phosphate according to any one of claims 1 to 8.

10. An ammonium salt compound, characterized in that The ammonium salt compound is shown in 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.

11. The use of the ammonium salt compound according to claim 10, characterized in that: The ammonium salt compound acts as a dispersing aid.

12. Use of the ammonium salt compound according to claim 10 in the preparation of lithium iron phosphate.

13. A positive electrode sheet, characterized in that: The positive electrode sheet includes a current collector and a positive electrode active layer disposed on a surface of the current collector, and a component of the positive electrode active layer includes the lithium iron phosphate as claimed in claim 9.

14. A secondary battery, characterized in that: The secondary battery comprises the lithium iron phosphate as claimed in claim 9 or the positive electrode sheet as claimed in claim 13 .

15. An electrical device, characterized in that: The electric device comprises the secondary battery as claimed in claim 14.

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