A multilayer gradient-coated modified lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery

Abstract

This invention discloses a multilayer gradient-coated modified lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery, belonging to the field of new lithium battery materials technology. The invention employs an aluminum-magnesium co-doped modified lithium iron phosphate matrix, with an inner layer using an acetylene black / graphene composite carbon layer to construct a high-speed electron conduction channel, and an outer layer coated with a lithium lanthanum phosphate solid electrolyte layer, forming a gradient composite structure of a doped core, a carbon conductive intermediate layer, and an ion-conducting outer layer. This effectively solves the technical pain points of traditional lithium iron phosphate batteries, such as low conductivity, poor low-temperature performance, rapid capacity decay over long cycles, and high interfacial impedance. The material of this invention exhibits a 0.1C initial discharge specific capacity ≥165mAh / g, excellent capacity retention at 10C high rates, significantly improved low-temperature discharge performance at -20℃, and a significantly enhanced cycle life. The preparation process is simple and controllable, suitable for large-scale mass production, and can be widely applied in power lithium batteries, energy storage lithium batteries, and other fields, meeting the requirements for new material pre-approval and filing in the new energy field.

Description

Technical Field

[0001] This invention relates to the fields of new lithium battery cathode materials and new energy storage technology, specifically to a multi-layer gradient coated modified lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery. Background Technology

[0002] Lithium iron phosphate (LFP) has become the mainstream cathode material for power batteries and energy storage batteries due to its advantages such as high safety performance, long cycle life, low cost, and no heavy metal pollution. However, existing commercially available LFP has inherent defects: its electronic conductivity and lithium-ion migration rate are relatively low, resulting in poor rate discharge performance and severe capacity decay at low temperatures; the conventional single carbon coating structure can only improve electronic conductivity and cannot reduce solid-liquid interface impedance, and the material structure is prone to collapse during long-term cycling, resulting in low capacity retention after thousands of cycles.

[0003] Existing modification techniques mostly employ single-element doping or single-carbon layer coating, resulting in limited modification dimensions and failing to simultaneously address structural stability, electronic conductivity, lithium-ion transport efficiency, and low-temperature performance. Therefore, this invention proposes a novel material system and preparation process combining multi-layer gradient composite coating and dual-element co-doping, simultaneously addressing the shortcomings of existing technologies at the microstructural level. Summary of the Invention

[0004] The purpose of this invention is to provide a multilayer gradient coated modified lithium iron phosphate cathode material, its preparation method, and a lithium-ion battery, overcoming the defects of existing technologies such as poor rate capability, low temperature difference, and rapid cycle decay.

[0005] This invention discloses a multilayer gradient-coated modified lithium iron phosphate cathode material, comprising a lithium iron phosphate matrix, an inner conductive carbon composite layer, and an outer lithium-ion solid electrolyte coating layer; the matrix is ​​co-doped with aluminum and magnesium, the inner layer is an acetylene black + graphene composite conductive carbon, and the outer layer is a lanthanum phosphate lithium-ion conductor layer.

[0006] Further limits the material ratio, coating thickness, and carbon material ratio, while providing supporting industrial preparation processes and lithium-ion batteries using this material.

[0007] Compared with the prior art, the present invention has the following advantages:

[0008] 1. Co-doping with aluminum and magnesium optimizes the crystal structure, suppresses lattice distortion during charging and discharging, and improves structural stability;

[0009] 2. The inner graphene / acetylene black composite carbon layer constructs a three-dimensional conductive network, significantly improving electronic conductivity;

[0010] 3. The outer lithium lanthanum phosphate solid electrolyte layer reduces electrode interface impedance, increases lithium-ion migration rate, and improves low-temperature and high-rate performance;

[0011] 4. The synergistic effect of the gradient multilayer structure results in significantly better capacity retention after thousands of cycles, low-temperature discharge capacity, and high-rate performance than ordinary commercial LFPs;

[0012] 5. The preparation process parameters are mild and the process is simple, making it compatible with existing mass production lines and highly feasible for industrialization. Attached Figure Description

[0013] Figure 1 This is a schematic cross-sectional view of the microstructure of the gradient multilayer coated lithium iron phosphate of the present invention. Detailed Implementation

[0014] Example 1

[0015] A multilayer gradient coated modified lithium iron phosphate cathode material, by mass parts:

[0016] The composition consists of 93 parts lithium iron phosphate matrix, 3 parts inner conductive carbon composite layer, 2 parts outer lithium lanthanum phosphate coating layer, and 0.3 parts aluminum-magnesium dopant.

[0017] The mass ratio of acetylene black to graphene is 3:1;

[0018] The carbon layer is 6–8 nm thick, and the solid electrolyte layer is 12–15 nm thick.

[0019] Preparation steps:

[0020] 1. The raw materials were mixed in molar ratio, ball-milled for 2 hours, and spray-dried to obtain the precursor;

[0021] 2. Pre-calcined at 350℃ for 4 hours in a nitrogen atmosphere, then held at 750℃ for 8 hours to obtain the doped LFP matrix;

[0022] 3. Mix the composite carbon source and sonicate for 1 hour, then carbonize at 500℃ for 2 hours;

[0023] 4. Coat with lithium lanthanum phosphate sol and stir for 3 hours, then heat-treat at 600℃ for 3 hours;

[0024] 5. Sieve through a 300-mesh sieve and vacuum dry to obtain the finished product.

[0025] Performance testing: 0.1C specific capacity 168mAh / g, 10C discharge specific capacity 142mAh / g, 1C cycle 1000 cycles capacity retention 93%, -20℃ low temperature 1C discharge specific capacity 125mAh / g.

[0026] Comparative Example

[0027] It uses ordinary single carbon-coated commercial lithium iron phosphate, without elemental doping or outer ionic conductor coating.

[0028] Test results: 0.1C specific capacity 160mAh / g, 10C only 120mAh / g, 1000 cycles retention 85%, -20℃ only 95mAh / g.

[0029] As can be seen from the comparison, the technical solution of this invention has significant creative improvements in rate capability, low temperature, and cycle life, and meets the conditions for patent authorization and rapid pre-examination.

[0030] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A multilayer gradient-coated modified lithium iron phosphate cathode material, characterized in that, It includes a lithium iron phosphate substrate, an inner conductive carbon composite layer coated on the surface of the lithium iron phosphate substrate, and an outer lithium-ion solid electrolyte coating layer coated on the outside of the inner conductive carbon composite layer. The lithium iron phosphate matrix is ​​doped with aluminum and magnesium. The inner conductive carbon composite layer is composed of acetylene black and graphene. The outer lithium-ion solid electrolyte coating layer is made of lithium lanthanum phosphate. Made of materials.

2. The multilayer gradient-coated modified lithium iron phosphate cathode material according to claim 1, characterized in that, The proportions by weight are as follows: 90-95 parts of lithium iron phosphate matrix; Inner conductive carbon composite layer, 2-5 parts; 1 to 3 parts of outer lithium-ion solid electrolyte coating layer; 0.1 to 0.5 parts of aluminum-magnesium composite dopant.

3. The multilayer gradient-coated modified lithium iron phosphate cathode material according to claim 1, characterized in that: The thickness of the inner conductive carbon composite layer is 5 nm to 10 nm. The thickness of the outer lithium-ion solid electrolyte coating layer is 10nm to 20nm.

4. The multilayer gradient-coated modified lithium iron phosphate cathode material according to claim 1, characterized in that, The mass ratio of acetylene black to graphene is 2.5:1 to 3.5:

1.

5. A method for preparing the multilayer gradient coated modified lithium iron phosphate cathode material according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1: Take ferrous sulfate heptahydrate, ammonium dihydrogen phosphate, and lithium hydroxide according to the formula, mix them in a molar ratio of 1:1:1.02-1.08, add aluminum nitrate and magnesium nitrate as doping sources, add deionized water and ball mill for dispersion for 1.5-2.5 hours, and spray dry to obtain the doped lithium iron phosphate precursor. Step 2: Place the precursor in a nitrogen inert atmosphere, pre-calcine at 330-370℃ for 3-5 hours, then raise the temperature to 730-770℃ and hold for 7-9 hours, and then cool naturally to obtain aluminum-magnesium doped lithium iron phosphate matrix powder. Step 3: Mix the doped lithium iron phosphate matrix with glucose, graphene dispersion and acetylene black and ultrasonically disperse for 0.5-1.5 h. After drying, carbonize at 480-520℃ for 1.5-2.5 h in an inert atmosphere to form an inner conductive carbon composite layer. Step 4: Prepare lithium lanthanum phosphate sol. Add the carbon-coated powder to the sol and stir at a constant temperature for 2.5 to 3.5 hours. After filtration and drying, heat treat at 580 to 620°C for 2.5 to 3.5 hours to form an outer solid electrolyte coating layer. Step 5: Crush, pass through a 250-350 mesh sieve, and vacuum dry to obtain the finished multi-layer gradient coated modified lithium iron phosphate cathode material.

6. A lithium-ion battery, characterized in that, It comprises a positive electrode, a negative electrode, a separator, an electrolyte, and a shell, wherein the positive electrode is prepared using the multilayer gradient coated modified lithium iron phosphate positive electrode material as described in any one of claims 1 to 4.

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