Layered lithium iron phosphate positive electrode material, preparation method thereof, positive plate and lithium ion battery

Abstract

The invention relates to the field of lithium ion batteries, and discloses a layered lithium iron phosphate positive electrode material, a preparation method thereof, a positive plate and a lithium ion battery. The layered lithium iron phosphate positive electrode material has the lithium iron phosphate inner core, and the carbon layer, the metal oxide layer and the conductive polymer layer which cover the outer surface of the inner core, and the content of carbon elements in the carbon layer, the content of metal elements in the metal oxide layer, the content of carbon elements in the conductive polymer layer or the content of N and S elements is reasonably adjusted, so that the lithium iron phosphate positive electrode material is obtained. And an excellent conductive network structure is formed in the layered lithium iron phosphate positive electrode material, so that the prepared layered lithium iron phosphate positive electrode material has excellent electrochemical performance.

Description

Technical Field

[0001] The present invention relates to the field of lithium-ion batteries, and particularly to a layered lithium iron phosphate cathode material, a preparation method thereof, a cathode sheet, and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries have high volumetric energy density and gravimetric energy density, and excellent cycling performance. They are internationally recognized as ideal energy sources today and are also a new generation of green high-energy batteries. They are widely used in the field of portable devices such as mobile phones and digital cameras, large mobile energy fields such as plug-in hybrid vehicles and pure electric vehicles, and fixed energy fields such as energy storage power stations and UPS.

[0003] The cathode materials used in lithium-ion batteries are mainly ternary materials and lithium iron phosphate. Among them, lithium iron phosphate has been widely used in the field of lithium batteries due to its excellent cycling performance, structural stability, safety, and low cost. However, the relatively low working voltage and energy density of lithium iron phosphate severely limit its development in cathode materials.

[0004] In the prior art, the lithium iron phosphate material is often modified to obtain a cathode material with relatively excellent electrochemical performance. For example, Patent CN115775886A discloses a preparation method of a lithium iron phosphate / carbon fiber composite material. This patent modifies lithium iron phosphate by using carbon fiber materials, solves the problems of easy agglomeration of material particles and poor compactness during the preparation of battery materials, and thus enables the lithium battery to have excellent electrochemical performance. Another example is that Patent CN118851128A discloses a modified lithium iron phosphate, a preparation method thereof, a cathode electrode sheet, and a secondary battery. This patent uses a carbon source and a nitrogen source to enable lithium iron phosphate to still have excellent electrochemical performance at low temperatures. However, the preparation methods of lithium iron phosphate in the prior art still have problems such as complex operation, expensive raw materials, inability to be promoted, and the lithium iron phosphate obtained by preparation cannot simultaneously have excellent cycling performance, working voltage, and energy density. Summary of the Invention

[0005] The purpose of the present invention is to overcome the problems in the prior art that lithium iron phosphate cannot simultaneously have excellent cycling performance and energy density, and to provide a layered lithium iron phosphate cathode material that can simultaneously have excellent cycling performance and energy density.

[0006] To achieve the above purpose, on the one hand, the present invention provides a layered lithium iron phosphate cathode material, which includes a lithium iron phosphate core and a carbon layer, a metal oxide layer, and a conductive polymer layer that are sequentially covered on the outer surface of the core;

[0007] Among them, in the layered lithium iron phosphate cathode material, the content of C element in the carbon layer is 1wt%-3wt%;

[0008] In the layered lithium iron phosphate cathode material, the content of the metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc, and manganese;

[0009] In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 wt% - 4 wt%; and / or, the content of S element is 2 wt% - 4 wt%.

[0010] The second aspect of the present invention provides a method for preparing a layered lithium iron phosphate cathode material, the method comprising:

[0011] S1: Prepare a first slurry containing a lithium iron phosphate raw material and a carbon source, place the first slurry in a hydrothermal reaction kettle for a first reaction, and then perform a first drying treatment on the first slurry to obtain a first precursor;

[0012] S2: Prepare a second slurry containing the first precursor and a metal source, and then perform a second drying treatment on the second slurry to obtain a second precursor;

[0013] S3: Prepare a third slurry containing the second precursor and a conductive source, and then perform a third drying treatment on the third slurry to obtain a third precursor;

[0014] S4: Roast the third precursor to prepare the layered lithium iron phosphate cathode material;

[0015] Wherein, the carbon source is selected from at least one of glucose, polyethylene, and polyethylene glycol;

[0016] The metal source is selected from at least one of aluminum oxide, zinc stannate, and manganese dioxide;

[0017] The conductive source is selected from at least one of polyaniline, polystyrene thiol, and polyacetylene;

[0018] The mass ratio of the carbon source, the metal source, and the conductive source is 1:1 - 5:0.5 - 4;

[0019] This method enables the prepared layered lithium iron phosphate cathode material to include a lithium iron phosphate core and a carbon layer, a metal oxide layer, and a conductive polymer layer sequentially covering the outer surface of the core;

[0020] Wherein, in the layered lithium iron phosphate cathode material, the content of carbon element in the carbon layer is 1 wt% - 3 wt%;

[0021] In the layered lithium iron phosphate cathode material, the content of the metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc, and manganese;

[0022] In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 - 4 wt%; and / or, the content of S element is 2 - 4 wt%.

[0023] The third aspect of the present invention provides a layered lithium iron phosphate cathode material prepared by the preparation method described in the second aspect of the present invention.

[0024] The fourth aspect of the present invention provides a positive electrode sheet, which is coated with the layered lithium iron phosphate cathode material described in the first aspect or the third aspect of the present invention.

[0025] The fifth aspect of the present invention provides a lithium ion battery, and the positive electrode sheet of the lithium ion battery is the positive electrode sheet described in the fourth aspect of the present invention.

[0026] The layered lithium iron phosphate cathode material prepared by the above technical solution has a lithium iron phosphate core and a carbon layer, a metal oxide layer and a conductive polymer layer covering the outer surface of the core. By reasonably adjusting the content of carbon element in the carbon layer, the content of metal element in the metal oxide layer, the content of carbon element or N element and S element in the conductive polymer layer, an excellent conductive network structure is formed in the layered lithium iron phosphate cathode material, and further the prepared layered lithium iron phosphate cathode material has excellent electrochemical performance. Detailed Embodiments

[0027] In the ranges disclosed herein, the endpoints and any values are not limited to the exact ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, between the endpoint values of each range, between the endpoint values of each range and a single point value, and between single point values, they can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0028] One aspect of the present invention provides a layered lithium iron phosphate cathode material, which includes a lithium iron phosphate core and a carbon layer, a metal oxide layer and a conductive polymer layer covering the outer surface of the core in sequence;

[0029] Among them, in the layered lithium iron phosphate cathode material, the content of C element in the carbon layer is 1 wt% - 3 wt%;

[0030] In the layered lithium iron phosphate cathode material, the content of metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc and manganese;

[0031] In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 wt% - 4 wt%; and / or, the content of S element is 2 wt% - 4 wt%.

[0032] In the present invention, reasonably controlling the composition of the carbon layer in the layered lithium iron phosphate cathode material can enable the prepared lithium-ion battery to have excellent electrochemical performance. Preferably, the content of C element in the carbon layer is 1.5 wt% - 2.8 wt%, for example, it can be 1.5 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.4 wt%, 2.6 wt%, 2.8 wt% and other values and the ranges between any of these values.

[0033] In the present invention, to further improve the electrochemical performance of the subsequently prepared lithium-ion battery, it can be achieved by regulating the content of metal elements in the metal oxide layer. Preferably, the content of metal elements in the metal oxide layer is 3.2 wt% - 4.6 wt%, for example, it can be 3.2 wt%, 3.4 wt%, 3.6 wt%, 3.8 wt%, 4.0 wt%, 4.2 wt%, 4.4 wt%, 4.6 wt% and other values and the ranges between any of these values.

[0034] In the present invention, reasonably regulating the composition of the conductive polymer layer in the layered lithium iron phosphate cathode material can also prepare a lithium-ion battery with excellent electrochemical performance. Preferably, the content of C element in the conductive polymer layer is 1.2 wt% - 2.5 wt%, for example, it can be 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2.0 wt%, 2.2 wt%, 2.4 wt%, 2.5 wt% and other values and the ranges between any of these values; and / or, the content of N element in the conductive polymer layer is 2.1 wt% - 3.7 wt%, for example, it can be 2.1 wt%, 2.3 wt%, 2.5 wt%, 2.8 wt%, 3.1 wt%, 3.3 wt%, 3.5 wt%, 3.7 wt% and other values and the ranges between any of these values; and / or, the content of S element in the conductive polymer layer is 2.5 wt% - 3.5 wt%, for example, it can be 2.5 wt%, 2.7 wt%, 2.9 wt%, 3.1 wt%, 3.3 wt%, 3.5 wt% and other values and the ranges between any of these values.

[0035] In the present invention, a lithium-ion battery with excellent electrochemical performance can be prepared by defining the size of the coating layer such as a carbon layer, a metal oxide layer or a conductive polymer layer in the layered lithium iron phosphate cathode material. Preferably, the thickness of the carbon layer is 1-20 nm, preferably 3-15 nm, for example, it can be values such as 3 nm, 6 nm, 9 nm, 12 nm, 15 nm and the ranges between any of these values.

[0036] In the present invention, preferably, the thickness of the metal oxide layer is 2-15 nm, preferably 4-12 nm, for example, it can be values such as 4 nm, 6 nm, 8 nm, 10 nm, 12 nm and the ranges between any of these values.

[0037] In the present invention, preferably, the thickness of the conductive polymer layer is 2-20 nm, preferably 5-16 nm, for example, it can be values such as 5 nm, 7 nm, 9 nm, 11 nm, 13 nm, 16 nm and the ranges between any of these values.

[0038] The second aspect of the present invention provides a method for preparing a layered lithium iron phosphate cathode material, the method comprising:

[0039] S1: Prepare a first slurry containing a lithium iron phosphate raw material and a carbon source, place the first slurry in a hydrothermal reaction kettle for a first reaction, and then perform a first drying treatment on the first slurry to obtain a first precursor;

[0040] S2: Prepare a second slurry containing the first precursor and a metal source, and then perform a second drying treatment on the second slurry to obtain a second precursor;

[0041] S3: Prepare a third slurry containing the second precursor and a conductive source, and then perform a third drying treatment on the third slurry to obtain a third precursor;

[0042] S4: Roast the third precursor to prepare the layered lithium iron phosphate cathode material;

[0043] wherein, the carbon source is selected from at least one of glucose, polyethylene and polyethylene glycol;

[0044] the metal source is selected from at least one of aluminum oxide, zinc stannate and manganese dioxide;

[0045] the conductive source is selected from at least one of polyaniline, polystyrene thiol and polyacetylene;

[0046] the mass ratio of the carbon source, the metal source and the conductive source is 1:1-5:0.5-4;

[0047] This method enables the prepared layered lithium iron phosphate cathode material to include a lithium iron phosphate core and a carbon layer, a metal oxide layer, and a conductive polymer layer that are sequentially covered on the outer surface of the core;

[0048] Among them, in the layered lithium iron phosphate cathode material, the content of C element in the carbon layer is 1 wt% - 3 wt%;

[0049] In the layered lithium iron phosphate cathode material, the content of metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc, and manganese;

[0050] In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 - 4 wt%; and / or, the content of S element is 2 - 4 wt%.

[0051] In the present invention, preferably, the mass ratio of the amounts of the carbon source, metal source, and conductive source used is 1:1.1 - 3.2:0.6 - 2.5.

[0052] In the present invention, preferably, based on the mass of the lithium iron phosphate raw material, the dosage of the carbon source is 1 - 10 wt%, preferably 6 - 8 wt%, and for example, it can be values such as 6 wt%, 7 wt%, 8 wt%, etc. and the ranges between any of these values.

[0053] In the present invention, preferably, the method for preparing the first slurry includes: placing the lithium iron phosphate raw material and the carbon source in deionized water for a first mixing treatment. Preferably, the conditions of the first mixing treatment include: the temperature is 20 - 40 °C, and the time is 1 - 3 h. More preferably, in the first mixing treatment, based on the total mass of the lithium iron phosphate raw material and the carbon source, the dosage of the deionized water is 80 - 120 wt%, preferably 90 - 110 wt%.

[0054] According to a preferred embodiment, the method for preparing the layered lithium iron phosphate cathode material further includes: performing a first grinding treatment on the first slurry; the conditions of the first grinding treatment include: the rotation speed is 200 - 300 r / min; the time is 6 - 10 h.

[0055] In the present invention, preferably, the conditions of the first reaction include: the temperature is 140 - 250 °C, preferably 150 - 200 °C, and the time is 5 - 12 h, preferably 8 - 10 h.

[0056] In the present invention, preferably, the conditions of the first drying treatment include: the temperature is 50 - 80 °C, preferably 55 - 75 °C, and the time is 1 - 5 h, preferably 2 - 3 h.

[0057] In the present invention, preferably, the method for preparing the second slurry includes: placing the first precursor and the metal source in deionized water for a second mixing treatment. Preferably, the conditions for the second mixing treatment include: a temperature of 30 - 50 °C and a time of 3 - 6 h. More preferably, in the second mixing treatment, based on the total mass of the lithium iron phosphate raw material and the metal source, the amount of deionized water used is 70 - 100 wt%, preferably 85 - 90 wt%.

[0058] According to a preferred embodiment, the method for preparing the layered lithium iron phosphate cathode material further includes: performing a second grinding treatment on the second slurry; the conditions for the second grinding treatment include: a rotation speed of 400 - 800 r / min; a time of 3 - 5 h.

[0059] In the present invention, preferably, the conditions for the second drying treatment include: a temperature of 60 - 90 °C, preferably 65 - 85 °C, and a time of 2 - 6 h, preferably 3 - 4 h.

[0060] In the present invention, the method for preparing the third slurry includes: placing the second precursor and the conductive source in deionized water for a third mixing treatment. Preferably, the conditions for the third mixing treatment include: a temperature of 25 - 45 °C and a time of 2 - 4 h. More preferably, in the third mixing treatment, based on the total mass of the lithium iron phosphate raw material and the conductive source, the amount of deionized water used is 60 - 120 wt%, preferably 70 - 80 wt%.

[0061] According to a preferred embodiment, the method for preparing the layered lithium iron phosphate cathode material further includes: performing a third grinding treatment on the third slurry; the conditions for the third grinding treatment include: a rotation speed of 1000 - 2000 r / min; a time of 5 - 8 h.

[0062] In the present invention, preferably, the conditions for the third drying treatment include: a temperature of 70 - 100 °C, preferably 75 - 95 °C, and a time of 3 - 7 h, preferably 4 - 5 h.

[0063] In the present invention, preferably, the conditions for the calcination treatment include: a temperature of 500 - 800 °C, preferably 600 - 700 °C, and a time of 3 - 10 h, preferably 5 - 8 h.

[0064] In the present invention, preferably, the lithium iron phosphate raw material includes a lithium source, an iron source, and a phosphorus source, and the lithium iron phosphate raw material optionally includes at least one of an aluminum source, a zinc source, and a manganese source.

[0065] In the present invention, in order to improve the coordination effect of the lithium iron phosphate raw material and prepare a lithium-ion battery with excellent electrochemical performance, preferably, the lithium iron phosphate prepared from the lithium iron phosphate raw material can have the chemical formula shown in Formula (1);

[0066] Formula (1): LiFe x M y PO4, where M is at least one of Al, Zn, and Mn;

[0067] where 0 < x < 0.8; 0 < y < 0.6, and x + y = 1.

[0068] In the present invention, in order to further improve the coordination effect of the lithium iron phosphate raw material and prepare a lithium-ion battery with excellent electrochemical performance, preferably, the lithium source is selected from at least one of Li2CO3, LiOH, and LiH2PO4.

[0069] In the present invention, preferably, the iron source is selected from at least one of FeSO4, FeC6H5O7, and FeC2O4.

[0070] In the present invention, preferably, the phosphorus source is selected from at least one of H3PO4, (NH4)3PO4, and NH4H2PO4.

[0071] In the present invention, preferably, the aluminum source is selected from at least one of Al2O3, Al(OH)3, and AlCl3.

[0072] In the present invention, preferably, the zinc source is selected from ZnO and / or ZnCl2.

[0073] In the present invention, preferably, the manganese source is selected from at least one of MnO, MnO2, and Mn2O3.

[0074] The third aspect of the present invention provides a layered lithium iron phosphate cathode material prepared by the preparation method described in the second aspect of the present invention.

[0075] The fourth aspect of the present invention provides a positive electrode sheet, which is coated with the layered lithium iron phosphate cathode material described in the first aspect or the third aspect of the present invention.

[0076] According to the present invention, the preparation method of the positive electrode can adopt various methods commonly used in the art. For example, it can include mixing the layered lithium iron phosphate cathode material, a positive electrode binder, and a positive electrode solvent proposed by the present invention, coating and / or filling them on a positive electrode current collector, forming a positive electrode material layer on the surface of the positive electrode current collector, drying, and calendering or not calendering to obtain the positive electrode. Preferably, in the positive electrode material layer of the battery, the content of the layered lithium iron phosphate cathode material provided by the present invention is 85-95 wt%.

[0077] In the present invention, the types of the positive electrode binder can be, for example, one or more of polypropylene, polyethylene, polyvinylidene fluoride, polyvinyl fluoride, vinylidene fluoride hexafluoropropylene, polytetrafluoroethylene, and polyhexafluoropropylene. Preferably, based on the layered lithium iron phosphate positive electrode material provided by the present invention, the amount of the positive electrode binder can be 0.01 - 5 wt%.

[0078] In the present invention, the positive electrode material layer preferably further contains a positive electrode conductive agent. The types of the positive electrode conductive agent can be, for example, one or several of conductive carbon black, carbon fiber, acetylene black, Ketjen black, and graphene. Preferably, based on the layered lithium iron phosphate positive electrode material provided by the present invention, the amount of the positive electrode conductive agent can be 0.01 - 5 wt%.

[0079] The fifth aspect of the present invention provides a lithium ion battery, and the positive electrode sheet of the lithium ion battery is the positive electrode sheet described in the fourth aspect of the present invention.

[0080] According to the present invention, there are no particular limitations on the negative electrode, separator, and non-aqueous electrolyte solution of the battery. Various types of negative electrodes, separators, and non-aqueous electrolytes used in the field of battery preparation (such as the field of lithium ion battery preparation) can be used, as long as the positive electrode adopted by the battery contains the layered lithium iron phosphate positive electrode material provided by the present invention.

[0081] The preparation method of the negative electrode can adopt various methods commonly used in the art. For example, it can include mixing a negative electrode active material, a negative electrode binder, and a negative electrode solvent, coating and / or filling them on a negative electrode current collector, forming a negative electrode material layer on the surface of the negative electrode current collector, drying, and calendering or not calendering to obtain the negative electrode.

[0082] In the present invention, the negative electrode active material can be selected from artificial graphite and / or natural graphite. Preferably, in the negative electrode material layer of the battery, the content of the negative electrode active material is 80 - 100 wt%.

[0083] According to the present invention, the negative electrode binder can be selected from one or several of polypropylene, polyethylene, polyvinylidene fluoride, vinyl chloride - hexafluoropropylene, polytetrafluoroethylene, and polyhexafluoropropylene. Preferably, based on the negative electrode active material, the amount of the negative electrode binder can be 0.01 - 5 wt%.

[0084] According to the present invention, the negative electrode material layer preferably further contains a negative electrode conductive agent. The negative electrode conductive agent can be one or several of conductive carbon black, carbon fiber, acetylene black, Ketjen black, graphene, and carbon nanotubes. Preferably, based on the negative electrode active material, the amount of the negative electrode conductive agent can be 0.01 - 5 wt%.

[0085] The types of current collectors in the positive electrode and the negative electrode are well-known to those skilled in the art and can be selected from any one of aluminum foil, copper foil, and punched steel strip, for example.

[0086] The separator is disposed between the positive electrode and the negative electrode and has electrical insulation performance and liquid retention performance. The separator can be various separators used in lithium-ion batteries, such as PP / PE separators, polyolefin microporous membranes, polyethylene felts, etc.

[0087] The non-aqueous electrolyte is a mixed solution of an electrolyte lithium salt and a non-aqueous solvent, and conventional non-aqueous electrolytes in the art can be used.

[0088] The preparation method of the battery is a conventional preparation method in the art. Generally, the positive electrode and the negative electrode are wound and separated by a separator layer to form an electrode group, and the obtained electrode group and the non-aqueous electrolyte are sealed in a battery case to obtain the battery provided by the present invention.

[0089] According to the present invention, the lithium-ion battery prepared by adopting the foregoing technical solution has excellent energy density and battery cycle performance. According to some preferred embodiments of the present invention, the lithium-ion battery prepared by using the layered lithium iron phosphate positive electrode material of the present invention has an energy density of not less than 190 wh / kg and a capacity retention rate of higher than 80% after 1000 cycles at 1C.

[0090] The present invention will be described in detail below through examples.

[0091] Example 1

[0092] The following examples are used to illustrate the preparation of the layered lithium iron phosphate positive electrode material.

[0093] S1: At room temperature (25 °C, the same below), lithium iron phosphate raw materials are weighed according to the molar ratio of lithium: iron: manganese: phosphorus = 1: 0.5: 0.5: 1: 740 g of Li2CO3, 360 g of FeC2O4, 217 g of MnO2, and 660 g of (NH4)2HPO4. Then, the lithium iron phosphate raw materials and 120 g of glucose are subjected to a first mixing treatment for 2.5 h in 2000 g of deionized water to prepare a first slurry. Then, the first slurry is ground at 250 r / min for 7 h and then placed in a hydrothermal reaction kettle at 180 °C for reaction for 8 h. Subsequently, a first drying treatment is carried out at 55 °C for 2 h to prepare a first precursor.

[0094] S2: At 30 °C, the first precursor and 140 g of Al2O3 are subjected to a second mixing treatment for 3 h in 1900 g of deionized water to prepare a second slurry. Then, the second slurry is ground at 400 r / min for 3 h, and then a second drying treatment is carried out at 65 °C for 3 h to prepare a second precursor.

[0095] S3: At room temperature, mix the second precursor with 80 g of polyaniline in 1600 g of deionized water for the third mixing treatment for 2 h to obtain a third slurry. Then grind the third slurry at 1200 r / min for 5 h, and then conduct the third drying treatment at 80 °C for 4 h to obtain a third precursor;

[0096] S4: Bake the third precursor at 600 °C for 5 h to obtain the layered lithium iron phosphate cathode material A1.

[0097] Example 2

[0098] The following examples are used to illustrate the preparation of the layered lithium iron phosphate cathode material.

[0099] S1: At room temperature (25 °C, the same below), weigh the lithium iron phosphate raw materials according to the molar ratio of lithium: iron: manganese: phosphorus = 1: 0.4: 0.6: 1: 740 g of Li2CO3, 288 g of FeC2O4, 260 g of MnO2, and 660 g of (NH4)2HPO4. Then mix the lithium iron phosphate raw materials with 140 g of glucose in 2100 g of deionized water for the first mixing treatment for 3 h to obtain a first slurry. Then grind the first slurry at 300 r / min for 8 h and then place it in a hydrothermal reaction kettle at 190 °C for reaction for 9 h. Subsequently, conduct the first drying treatment at 60 °C for 2.5 h to obtain a first precursor;

[0100] S2: At 35 °C, mix the first precursor with 200 g of Al2O3 in 1800 g of deionized water for the second mixing treatment for 4 h to obtain a second slurry. Then grind the second slurry at 500 r / min for 3 h, and then conduct the second drying treatment at 70 °C for 3 h to obtain a second precursor;

[0101] S3: At 30 °C, mix the second precursor with 200 g of polyaniline in 1700 g of deionized water for the third mixing treatment for 3 h to obtain a third slurry. Then grind the third slurry at 1500 r / min for 6 h, and then conduct the third drying treatment at 85 °C for 4 h to obtain a third precursor;

[0102] S4: Bake the third precursor at 650 °C for 6 h to obtain the layered lithium iron phosphate cathode material A2.

[0103] Example 3

[0104] According to a method similar to that of Example 1, the difference is that polyethylene glycol of the same weight part is used to replace glucose to obtain the layered lithium iron phosphate cathode material A3.

[0105] Example 4

[0106] According to the method similar to that of Example 1, except that ZnSnO3 with the same weight fraction is used to replace Al2O3, the layered lithium iron phosphate cathode material A4 is prepared.

[0107] Example 5

[0108] According to the method similar to that of Example 1, except that polystyrene thiol with the same weight fraction is used to replace polyaniline, the layered lithium iron phosphate cathode material A5 is prepared.

[0109] Example 6

[0110] According to the method similar to that of Example 1, 140 g of glucose is used. Except that the mass ratio of the amounts of glucose, Al2O3 and polyaniline used is 1:1:0.5, the layered lithium iron phosphate cathode material A6 is prepared.

[0111] Example 7

[0112] According to the method similar to that of Example 1, 140 g of glucose is used. Except that the mass ratio of the amounts of glucose, Al2O3 and polyaniline used is 1:5:4, the layered lithium iron phosphate cathode material A7 is prepared.

[0113] Comparative Example 1

[0114] The following examples are used to illustrate the preparation of the layered lithium iron phosphate cathode material.

[0115] S1: At room temperature (25 °C, the same below), according to the molar ratio of lithium: iron: manganese: phosphorus = 1:0.5:0.5:1, weigh the lithium iron phosphate raw materials: 740 g of Li2CO3, 360 g of FeC2O4, 217 g of MnO2 and 660 g of (NH4)2HPO4. Then, the lithium iron phosphate raw materials and 120 g of glucose are subjected to a first mixing treatment for 2.5 h in 2000 g of deionized water to prepare a first slurry. Then, the first slurry is ground at 250 r / min for 7 h and then placed in a hydrothermal reaction kettle at 180 °C for reaction for 8 h. Subsequently, a first drying treatment is carried out at 55 °C for 2 h to prepare a first precursor.

[0116] S2: At 30 °C, the first precursor and 140 g of Al2O3 are subjected to a second mixing treatment for 3 h in 1900 g of deionized water to prepare a second slurry. Then, the second slurry is ground at 400 r / min for 3 h, and then a second drying treatment is carried out at 65 °C for 3 h to prepare a second precursor.

[0117] S3: The second precursor is calcined at 600 °C for 5 h to prepare the layered lithium iron phosphate cathode material B1.

[0118] Comparative Example 2

[0119] According to the method similar to that of Example 1, 140 g of glucose was used. The difference is that the mass ratio of the amounts of glucose, Al2O3, and polyaniline is 1:0.5:0.1, and the layered lithium iron phosphate cathode material B2 was prepared.

[0120] Comparative Example 3

[0121] According to the method similar to that of Example 1, 140 g of glucose was used. The difference is that the mass ratio of the amounts of glucose, Al2O3, and polyaniline is 1:8:6, and the layered lithium iron phosphate cathode material B3 was prepared.

[0122] Test Example 1

[0123] The thickness of each coating layer and the content of each element in the layered lithium iron phosphate cathode materials prepared in the examples and comparative examples were measured by TEM and other methods. The specific results are shown in Table 1.

[0124] Table 1

[0125]

[0126] Test Example 2

[0127] The above-mentioned layered lithium iron phosphate cathode materials were prepared into lithium-ion batteries according to the following steps, and the electrochemical performance of the batteries was tested. The test results are shown in Table 2.

[0128] Preparation steps of the lithium-ion battery:

[0129] (1) Preparation of the positive electrode sheet: 22 g of the layered lithium iron phosphate cathode material prepared in the above examples and comparative examples, 0.6 g of conductive carbon black, and 0.9 g of polyvinylidene fluoride were mixed and placed in a solvent, and then ball-milled at a speed of 360 rmp for 4 h to obtain a slurry. The slurry was coated on an aluminum foil and dried to obtain the positive electrode sheet;

[0130] (2) Preparation of the negative electrode sheet: 30 g of natural graphite, 0.2 g of conductive carbon black, and 0.5 g of polyvinylidene fluoride were mixed and placed in a solvent, and then ball-milled at a speed of 400 rmp for 5 h to obtain a slurry. The slurry was coated on an aluminum foil and dried to obtain the negative electrode sheet;

[0131] (3) Assembly of the lithium-ion battery: In a glove box, the negative electrode sheet, PP / PE separator, and positive electrode sheet were assembled. During the process, a lithium salt electrolyte was injected, and then the coin cell was sealed with a sealer. The assembled coin cell was used for subsequent electrochemical performance testing.

[0132] Test method:

[0133] (1) Energy density test method: Accurately weigh the mass of the prepared battery cell (unit: g); through 1C charge-discharge cycles, measure the actual discharge capacity of the battery cell (unit: Ah); record the rated voltage of the battery cell (unit: volts, V); calculate the energy of the battery cell using the measured capacity and voltage, and then divide the energy by the mass of the battery cell to obtain the mass energy density (unit: Wh / kg).

[0134] (2) Cycle performance test method: Perform constant current-constant voltage charging at a 1C rate, after standing for 10 min, perform constant current discharge at a 1C rate, record the number of cycles during this process and compare the discharge capacity corresponding to each cycle with the discharge capacity of the first cycle.

[0135] Table 2

[0136]

[0137]

[0138] From the results in Table 1, it can be seen that compared with Comparative Examples 1-3, the lithium-ion battery prepared by the preparation method of the present invention has excellent energy density and cycle performance. In particular, the energy density of the lithium-ion battery prepared in Examples 1-5 is not less than 200 Wh / kg, and the capacity retention rate after 1000 cycles is not less than 85%.

[0139] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited thereto. Within the technical concept scope of the present invention, various simple modifications can be made to the technical solutions of the present invention, including any other suitable combination of each technical feature. These simple modifications and combinations should also be regarded as the content disclosed by the present invention and fall within the protection scope of the present invention.

Claims

1. A layered lithium iron phosphate cathode material, characterized in that, The material includes a lithium iron phosphate core and a carbon layer, a metal oxide layer, and a conductive polymer layer that successively cover the outer surface of the core; Among them, in the layered lithium iron phosphate cathode material, the content of C element in the carbon layer is 1 wt% - 3 wt%; In the layered lithium iron phosphate cathode material, the content of metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc, and manganese; In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 wt% - 4 wt%; and / or, the content of S element is 2 wt% - 4 wt%.

2. The layered lithium iron phosphate cathode material according to claim 1, wherein, The content of C element in the carbon layer is 1.5 wt% - 2.8 wt%; Preferably, the content of metal element in the metal oxide layer is 3.2 wt% - 4.6 wt%.

3. The layered lithium iron phosphate cathode material according to claim 1 or 2, wherein The content of C element in the conductive polymer layer is 1.2 wt% - 2.5 wt%; and / or, the content of N element in the conductive polymer layer is 2.1 wt% - 3.7 wt%; and / or, the content of S element in the conductive polymer layer is 2.5 wt% - 3.5 wt%.

4. The layered lithium iron phosphate cathode material according to any one of claims 1-3, wherein, The thickness of the carbon layer is 1 - 20 nm, preferably 3 - 15 nm; Preferably, the thickness of the metal oxide layer is 2 - 15 nm, preferably 4 - 12 nm; Preferably, the thickness of the conductive polymer layer is 2 - 20 nm, preferably 5 - 16 nm.

5. A preparation method of a layered lithium iron phosphate cathode material, characterized in that, The method includes: S1: Prepare a first slurry containing a lithium iron phosphate raw material and a carbon source, place the first slurry in a hydrothermal reactor for a first reaction, and then perform a first drying treatment on the first slurry to obtain a first precursor; S2: Prepare a second slurry containing the first precursor and a metal source, and then perform a second drying treatment on the second slurry to obtain a second precursor; S3: Prepare a third slurry containing the second precursor and a conductive source, and then perform a third drying treatment on the third slurry to obtain a third precursor; S4: Calcinate the third precursor to prepare the layered lithium iron phosphate cathode material; Among them, the carbon source is selected from at least one of glucose, polyethylene, and polyethylene glycol; The metal source is selected from at least one of aluminum oxide, zinc stannate, and manganese dioxide; The conductive source is selected from at least one of polyaniline, polystyrene thiol, and polyacetylene; The mass ratio of the dosages of the carbon source, metal source, and conductive source is 1:1 - 5:0.5 - 4; This method enables the prepared layered lithium iron phosphate cathode material to include a lithium iron phosphate core and a carbon layer, a metal oxide layer, and a conductive polymer layer that successively cover the outer surface of the core; Among them, in the layered lithium iron phosphate cathode material, the content of C element in the carbon layer is 1 wt% - 3 wt%; In the layered lithium iron phosphate cathode material, the content of metal element in the metal oxide layer is 3 wt% - 5 wt%; the metal is selected from at least one of aluminum, zinc, and manganese; In the layered lithium iron phosphate cathode material, the content of C element in the conductive polymer layer is 1 wt% - 3 wt%; and / or, the content of N element is 2 wt% - 4 wt%; and / or, the content of S element is 2 wt% - 4 wt%.

6. The preparation method according to claim 5, wherein, The mass ratio of the amounts of the carbon source, metal source, and conductive source used is 1: 1.1 - 3.2: 0.6 - 2.

5.

7. The preparation method according to claim 5 or 6, wherein The conditions of the first reaction include: the temperature is 140 - 250 °C, preferably 150 - 200 °C, and the time is 5 - 12 h, preferably 8 - 10 h; The conditions of the first drying treatment include: the temperature is 50 - 80 °C, preferably 55 - 75 °C, and the time is 1 - 5 h, preferably 2 - 3 h; Preferably, the conditions of the second drying treatment include: the temperature is 60 - 90 °C, preferably 65 - 85 °C, and the time is 2 - 6 h, preferably 3 - 4 h; Preferably, the conditions of the third drying treatment include: the temperature is 70 - 100 °C, preferably 75 - 95 °C, and the time is 3 - 7 h, preferably 4 - 5 h; Preferably, the conditions of the calcination treatment include: the temperature is 500 - 800 °C, preferably 600 - 700 °C, and the time is 3 - 10 h, preferably 5 - 8 h.

8. The preparation method according to any one of claims 5-7, wherein, The lithium iron phosphate raw material includes a lithium source, an iron source, and a phosphorus source, and the lithium iron phosphate raw material optionally includes at least one of an aluminum source, a zinc source, and a manganese source; Preferably, the lithium source is selected from at least one of Li2CO3, LiOH, and LiH2PO4; Preferably, the iron source is selected from at least one of FeSO4, FeC6H5O7, and FeC2O4; Preferably, the phosphorus source is selected from at least one of H3PO4, (NH4)3PO4, and NH4H2PO4; Preferably, the aluminum source is selected from at least one of Al2O3, Al(OH)3, and AlCl3; Preferably, the zinc source is selected from ZnO and / or ZnCl2; Preferably, the manganese source is selected from at least one of MnO, MnO2, and Mn2O3.

9. A layered lithium iron phosphate cathode material prepared by the preparation method according to any one of claims 5 - 8.

10. A positive electrode sheet, characterized in that, The positive electrode sheet is coated with the layered lithium iron phosphate cathode material according to any one of claims 1 - 4, 9.

11. A lithium-ion battery, characterized in that, The positive electrode sheet of the lithium ion battery is the positive electrode sheet according to claim 10.

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