A nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material and preparation method thereof

Abstract

The invention discloses a method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material, wherein nitrogen and phosphine gas are used as nitrogen source and phosphorus source respectively, and a graphite target is sputtered by unbalanced magnetron sputtering technology, and a nitrogen-phosphorus co-doped carbon film is coated on the surface of the lithium titanate negative electrode material, wherein the nitrogen doping amount is not less than 5.0 wt%, the phosphorus doping amount is not less than 2.0 wt%, and the carbon content of the Sp2 structure is at least 70 at%. Phosphorus-nitrogen co-doping can effectively improve the conductivity and Li+ diffusion coefficient of the lithium titanate electrode material, and after being prepared into a lithium ion half-cell, it shows good rate performance. In addition, the present invention utilizes unbalanced magnetron sputtering technology, does not involve a high temperature process, effectively avoids grain growth and unnecessary energy loss, and simultaneously ensures a high doping amount, reduces the preparation cost, and the preparation process has high controllability, high efficiency, low pollution, and has broad market application prospects.

Description

Technical Field

[0001] The invention relates to the preparation of lithium titanate negative electrode materials, in particular to a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material and a preparation method thereof, belonging to the fields of composite materials and lithium ion battery technology. Background Art

[0002] Secondary batteries represented by lithium-ion batteries can operate stably for many years or decades as new energy storage devices, and are the key to the large-scale application of intermittent renewable energy. The stable discharge platform of spinel Li4Ti5O12 is about 1.55 V (vsLi / Li+), which is higher than the reduction potential of most electrolyte solvents, so there is no SEI film formation, which is a safer negative electrode material. At the same time, the high voltage eliminates the possibility of lithium plating. Furthermore, Li4Ti5O12 has very small structural changes (zero strain) during lithium insertion / extraction, which ensures good reversibility and structural stability in long cycles, and has excellent market application prospects. However, the main difficulty in using this type of material is that the conductivity (ca.10-13 S cm-1) and Li+ diffusion coefficient (10-9~10-13 cm2 s-1) are very low, which cannot meet the application at high rates.

[0003] Surface coating of lithium titanate electrode materials to promote electron transmission, reduce charge transfer resistance, thereby increasing ion diffusion rate and improving electrochemical performance is the best choice at present. CN114944483A discloses a method for modifying lithium titanate negative electrode materials. The preparation method comprises the following steps: a. mixing titanium salt with anhydrous ethanol containing glacial acetic acid to obtain titanium sol, adding lithium acetate and carbon source to the titanium sol, stirring until clarified, and obtaining lithium titanate sol; b. performing vacuum distillation on the lithium titanate sol obtained in step a under ultraviolet light irradiation to obtain lithium titanate precursor powder; c. calcining the lithium titanate precursor powder obtained in step b to obtain a carbon-coated spinel structure lithium titanate negative electrode material; ... 7269A discloses a method for preparing a carbon-coated lithium titanate electrode material, comprising the steps of: taking a titanium foil, placing it in a hydrothermal kettle, adding a hydrothermal reagent (a hydrothermal reagent containing an aqueous solution of lithium ions, an etchant and an organic alcohol), and hydrothermally reacting at 160°C to 200°C for 4h to 8h to obtain a flaky lithium titanate@polymer precursor composite material; then placing the precursor composite material in a tubular furnace, while introducing an inert gas, annealing at 600°C to 900°C for 1h to 3h, and cooling to obtain a flaky carbon-coated lithium titanate electrode material. However, the above schemes all require high-temperature treatment during the formation of the film, which is prone to phase change and grain growth of the electrode material, destroying the original structure. At the same time, the process is complicated and energy-consuming. Therefore, it is very necessary to develop a low-temperature nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material and its preparation method. Summary of the invention

[0004] In view of the above technical problems, the present invention provides a method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material to improve the conductivity of the lithium titanate negative electrode material and avoid the growth of lithium titanate grains and unnecessary energy loss.

[0005] The present invention discloses a method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material, which uses nitrogen gas and phosphine gas as nitrogen source and phosphorus source respectively, sputters a graphite target material by using an unbalanced magnetron sputtering technology, and coats a nitrogen-phosphorus co-doped carbon film on the surface of the lithium titanate negative electrode material. The specific process includes the following steps:

[0006] (1) After vacuum drying, the lithium titanate negative electrode powder is placed on the rotating frame of the vacuum coating chamber of the magnetron sputtering device, and the target spacing is adjusted and the vacuum is evacuated to 10 -4 Pa, turn on the rotating rack and graphite target, and make the graphite target rotate at a constant speed in the opposite direction to the rack. Vacuum drying is carried out in a vacuum oven at 110°C for 120 min to remove the moisture and some volatile organic matter adsorbed on the surface of the material;

[0007] (2) Argon, nitrogen, and phosphine gases were introduced, and the chamber working pressure was adjusted to 1.1 Pa. The nitrogen-phosphorus co-doped carbon film was deposited using unbalanced magnetron sputtering technology. The argon flow rate was maintained at 50 sccm, and the gas flow ratio of nitrogen and phosphine gases was 5:1~2:1; the current of the graphite target was 1A, and the output power was 800W; the target thickness of the nitrogen-phosphorus co-doped carbon film was 10nm.

[0008] XPS analysis shows that the nitrogen doping amount in the prepared nitrogen-phosphorus co-doped carbon film is not less than 5.0 wt%, the phosphorus doping amount is not less than 2.0 wt%, and the carbon content of the Sp2 structure is at least 70 at%.

[0009] Figure 1 This is a schematic diagram of the structure of the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material prepared by the present invention. A nitrogen-phosphorus co-doped carbon film is obtained on the surface of the lithium titanate electrode material. Phosphorus doping can promote the conversion of Sp3 carbon to Sp2 carbon, allowing the carbon content of the Sp2 structure to reach 70at%, showing graphite-like properties. Therefore, the phosphorus-doped carbon film can effectively improve the electrical conductivity of the material. Nitrogen doping can give the carbon film a greater electron mobility and more exposed active sites, which can provide better electrochemical behavior and can effectively improve the electrical conductivity and Li+ diffusion coefficient of the lithium titanate electrode material. Figure 2 and 3 As shown, after treatment, a large number of nanostructures are formed on the surface of lithium titanate, which are composed of nitrogen and phosphorus co-doped graphite structured carbon.

[0010] The prepared nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was prepared into a lithium-ion half-cell, which showed a large reversible specific capacity at high current densities of 5C and 10C in a secondary electrolyte (1.0M LiPF6 in EC:DEC:EMC=1:1:1 Vol%). Therefore, it has broad market application prospects as a negative electrode material for lithium-ion batteries.

[0011] In summary, the present invention has the following beneficial effects compared with the prior art:

[0012] 1. In the process of preparing carbon-coated lithium iron phosphate negative electrode materials, the present invention adopts a nitrogen and phosphorus co-doping method to solve the problems of low electronic conductivity and ion diffusion rate of lithium titanate negative electrode materials, while ensuring a high doping amount and reducing the preparation cost;

[0013] 2. The preparation process of the present invention does not involve high-temperature processes, which effectively avoids grain growth and unnecessary energy loss, while ensuring a high doping amount and reducing the preparation cost; the preparation process is highly controllable, efficient, and less polluting, and has broad market application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is a schematic diagram of the structure of the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material prepared in the present invention.

[0015] Figure 2 This is a SEM image of the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material prepared in Example 1 of the present invention.

[0016] Figure 3 This is a TEM image of the nitrogen and phosphorus co-doped carbon-coated lithium titanate negative electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0017] The preparation method and performance of the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material of the present invention are described in detail below through specific examples.

[0018] Example 1

[0019] (1) The lithium titanate raw material was dried in an oven at 110°C under vacuum for 120 min, 150 g was weighed, and placed in the rotating frame of the vacuum chamber in the magnetron sputtering device. The target spacing was adjusted and the vacuum was evacuated to 10 -4 Pa, turn on the rotating device, and make the graphite target and the work stand rotate at a constant speed in the opposite direction;

[0020] (2) Argon (50 sccm), nitrogen, and phosphine gas (gas flow ratio of 5:1) were introduced, the chamber working pressure was maintained at 1.1 Pa, the current of the graphite target was 1A, the output power was 800W, and the nitrogen-phosphorus co-doped carbon film was sputtered using unbalanced magnetron sputtering technology. The film thickness was 10 nm, and the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was obtained;

[0021] (3) The lithium titanate negative electrode material was prepared into a lithium-ion half-cell, which exhibited a reversible specific capacity of 140 mA h g-1 and 120 mA h g-1 at high current densities of 5C and 10C, respectively.

[0022] Example 2

[0023] (1) The lithium titanate raw material was dried in an oven at 110°C under vacuum for 120 min, 150 g was weighed, and placed in the rotating frame of the vacuum chamber in the magnetron sputtering device. The target spacing was adjusted and the vacuum was evacuated to 10 -4 Pa, turn on the rotating device, and make the graphite target and the work stand rotate at a constant speed in the opposite direction;

[0024] (2) Argon (50 sccm), nitrogen, and phosphine gas (gas flow ratio of 2:1) were introduced, the chamber working pressure was maintained at 1.1 Pa, the current of the graphite target was 1A, the output power was 800W, and the nitrogen-phosphorus co-doped carbon film was sputtered using unbalanced magnetron sputtering technology. The film thickness was 10 nm, and the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was obtained;

[0025] (3) The nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was prepared into a lithium-ion half-cell, which exhibited a reversible specific capacity of 130 mAhg-1 and 112 mA hg-1 at high current densities of 5C and 10C, respectively.

[0026] Example 3

[0027] (1) The lithium titanate raw material was dried in an oven at 110°C under vacuum for 120 min, 150 g was weighed, and placed in the rotating frame of the vacuum chamber in the magnetron sputtering device. The target spacing was adjusted and the vacuum was evacuated to 10 -4 Pa, turn on the rotating device, and make the graphite target and the work stand rotate at a constant speed in the opposite direction;

[0028] (2) Argon (50 sccm), nitrogen, and phosphine gas (gas flow ratio of 4:1) were introduced, the chamber working pressure was maintained at 1.1 Pa, the current of the graphite target was 1A, the output power was 800W, and the nitrogen-phosphorus co-doped carbon film was sputtered using unbalanced magnetron sputtering technology. The film thickness was 10 nm, and the nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was obtained;

[0029] (3) The nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material was prepared into a lithium-ion half-cell, which exhibited a reversible specific capacity of 135 mAhg-1 and 116 mA hg-1 at high current densities of 5C and 10C, respectively.

[0030] Comparative example: The lithium titanate negative electrode raw material without surface coating treatment was prepared into a lithium ion half-cell, which exhibited a reversible specific capacity of 70 mA hg-1 and 20 mA hg-1 at high current densities of 5C and 10C, respectively.

Claims

1. A nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material, which is obtained by sputtering a graphite target using an unbalanced magnetron sputtering technique with nitrogen gas and phosphine gas as a nitrogen source and a phosphorus source, respectively, to coat a layer of nitrogen-phosphorus co-doped carbon film on the surface of the lithium titanate negative electrode material; in the nitrogen-phosphorus co-doped carbon film, the nitrogen doping amount is not less than 5.0 wt%, the phosphorus doping amount is not less than 2.0 wt%, and Sp 2 The carbon content of the structure is at least 70 at%.

2. The method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material according to claim 1, comprising the following process steps: (1) After vacuum drying, place the lithium titanate negative electrode powder on the rotating frame of the vacuum coating chamber of the magnetron sputtering device, adjust the target spacing and evacuate to 10 -4 Pa, turn on the rotating workbench and graphite target; (2) Argon, nitrogen and phosphine gases were introduced, and the working pressure of the chamber was adjusted to be maintained at 1.1 Pa. The nitrogen-phosphorus co-doped carbon film was deposited using unbalanced magnetron sputtering technology.

3. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: In step (1), vacuum drying is performed in a vacuum oven at 110° C. for 120 min.

4. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: In step (1), the graphite target and the work frame rotate at a constant speed in opposite directions.

5. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: In step (2), the argon gas flow rate was maintained at 50 sccm.

6. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: In step (2), the gas flow ratio of nitrogen gas to phosphine gas is 5:1 to 2:

1.

7. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: In step (2), the current of the graphite target is 1A and the output power is 800W.

8. A method for preparing a nitrogen-phosphorus co-doped carbon-coated lithium titanate negative electrode material as claimed in claim 2, Features: The target thickness of the nitrogen-phosphorus co-doped carbon film is 10 nm.

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